Advertisement

Predicting the difficulty of operative vaginal delivery by ultrasound measurement of fetal head station

Published:January 29, 2017DOI:https://doi.org/10.1016/j.ajog.2017.01.007

      Background

      Clinical assessment of fetal head station is difficult and subjective; it is mandatory before attempting operative vaginal delivery.

      Objective

      The principal objective of our study was to assess whether measurement of the perineum-to-skull ultrasound distance was predictive of a difficult operative vaginal delivery. Secondary objectives included evaluation of the interobserver reproducibility of perineum-to-skull ultrasound distance and comparison of this measurement and digital examination in predicting a difficult operative delivery.

      Study Design

      This was a prospective cohort study including all cases of operative vaginal deliveries in singleton pregnancies in cephalic presentation >34 weeks’ gestation, from 2012 through 2015. All data were entered prospectively in a medical record system specially devised to meet the requirements of this study.

      Results

      Of the 659 patients in whom perineum-to-skull ultrasound distance was measured prior to operative vaginal delivery, 120 (18%) met the composite criterion for a difficult extraction. Perineum-to-skull ultrasound distance measurement of ≥40 mm was significantly associated with the occurrence of a difficult extraction based on the composite criterion, after adjustment for parity, presentation type, and fetal macrosomia (odds ratio, 2.38; 95% confidence interval, 1.51–3.74; P = .0002). The intraclass correlation coefficient between the perineum-to-skull ultrasound distance measured by the first operator and that measured by the second operator was 0.96 (95% confidence interval, 0.95–0.97; P < .0001). Based on the receiver operating characteristic curve analyses, perineum-to-skull ultrasound distance was a more accurate predictor of difficult operative delivery than digital vaginal examination (P = .036).

      Conclusion

      Measurement of the perineum-fetal skull ultrasound distance is a reproducible and predictive index of the difficulty of instrumental extraction. Ultrasound is a useful supplementary tool to the usual clinical findings.

      Key words

      Introduction

      Operative vaginal deliveries account on average for 13% of births in France but ranges from 8-24% of deliveries.

      Zettlin J, Mohangoo A, Delnord M. Health and care of pregnant women and babies in Europe in 2010. In: European Perinatal Health Report, ed. Belgium: European Perinatal Health Report. 2012:77-80.

      Blondel B, Kermarec M. Les naissances en 2010 et leur évolution depuis 2003. In: INSERM, ed. Paris, France: Enquête Nationale Périnatale 2010. 2011.

      When operative vaginal delivery is imminent on complete cervical dilatation, it is essential to know whether the fetus is engaged in the pelvis; when it is, assessment of the level of fetal head engagement is a significant factor in deciding on the appropriate mode of delivery.
      • Vayssière C.
      • Beucher G.
      • Sentilhes L.
      • et al.
      Instrumental delivery: clinical practice guidelines from the French College of Gynecologists and Obstetricians.
      Clinical diagnosis of fetal head engagement is difficult since the various signs conventionally used are subjective. There are discrepancies between the abdominal signs and information relayed by digital vaginal examination, these discrepancies being principally attributed to the fact that the presence of caput succedaneum may distort assessment of the fetal head position.
      • Knight D.
      • Newnham J.P.
      • McKenna M.
      • Evans S.
      A comparison of abdominal and vaginal examinations for the diagnosis of engagement of the fetal head.
      There are also discrepancies between findings relayed by digital vaginal examination and true fetal head station. This was the outcome of the study by Dupuis et al,
      • Dupuis O.
      • Gaucherand P.
      • Cucherat M.
      • et al.
      Birth simulator: reliability of transvaginal assessment of fetal head station as defined by the American College of Obstetricians and Gynecologists classification.
      which compared fetal head station as assessed by gynecologists (on digital vaginal examination of a birth simulator) to actual fetal head station: it found error rates in interpreting fetal head station of 20% when assessment of head station was described as high, mid, or low in the maternal pelvis. The error rate rose to 80% when it was described using the American Congress of Obstetricians and Gynecologists (ACOG) classification, which scores the fetal head station from –5 to +5 (0 corresponds to the level of the ischial spines).
      • Cunningham F.
      • MacDonald P.C.
      • Gant N.F.
      • et al.
      Conduct of normal labor and delivery.
      Several studies have assessed the utility of ultrasonography in measuring the degree of fetal head engagement. A recent study showed that ultrasound measurement of the angle of fetal head progression was more reliable than digital vaginal examination in predicting vaginal operative delivery failure.
      • Bultez T.
      • Quibel T.
      • Bouhanna P.
      • Popowski T.
      • Resche-Rigon M.
      • Rozenberg P.
      Angle of fetal head progression measured using transperineal ultrasound as a predictive factor of vacuum extraction failure.
      A linear method for measuring the perineum-to-skull ultrasound distance (PSUD) has also been proposed and is attractive because it is more straightforward to apply.
      • Fouché C.J.
      • Simon E.G.
      • Potin J.
      • Perrotin F.
      Ultrasound in monitoring of the second stage of labor.
      This method has, however, only been evaluated in limited-series studies and few data are available to confirm whether it is reproducible.
      • Eggebø T.M.
      • Gjessing L.K.
      • Romundstad P.
      • et al.
      Prediction of labor and delivery by transperineal ultrasound in pregnancies with prelabor rupture of membranes at term.
      • Maticot-Baptista D.
      • Ramanah R.
      • Collin A.
      • Martin A.
      • Maillet R.
      • Riethmuller D.
      Ultrasound in the diagnosis of fetal head engagement. A preliminary French prospective study.
      • Magnard C.
      • Perrot M.
      • Fanget C.
      • Paviot-Trombert B.
      • Raia-Barjat T.
      • Chauleur C.
      Extraction instrumentale sur une hauteur de présentation supérieur à 55 mm à l’échographie transpérinéale.
      The principal objective of our study was to assess whether ultrasound measurement of the PSUD was predictive of a difficult operative vaginal delivery and to define an optimum threshold for prediction of this event.
      Secondary objectives were to assess the interobserver reproducibility of this ultrasound measurement, to evaluate the correlation between data obtained by digital vaginal examination and ultrasound, as well as to compare the utility of transperineal ultrasound and digital vaginal examination in predicting a difficult operative vaginal delivery.

      Materials and Methods

      Experimental design

      This was a prospective cohort study conducted in the delivery room service of the medical-surgical and obstetrical center, part of the Strasbourg University Hospitals (level IIA), from November 2012 through April 2015.

      Eligibility criteria

      We included all cases of operative vaginal deliveries in singleton pregnancies in cephalic presentation >34 weeks’ gestation. All live births >34 weeks were therefore enrolled, whereas multiple pregnancies, noncephalic presentations, spontaneous vaginal deliveries, cesarean deliveries, and patients without measurement of the PSUD were excluded.

      Data collection

      Biometric, obstetrical, and neonatal data were entered prospectively in the Diamm electronic medical record system (Diamm, Micro6, Vandoeuvre Les Nancy, France) based on a specific collation method specially devised to meet the requirements of this study. Informed consent was obtained from all patients. The study was registered with the French data protection agency, National Commission on Informatics and Liberty, under registration number 1759739v0.

      Assessment criteria

      The principal assessment criterion was the difficulty of operative vaginal delivery based on a composite criterion termed “extraction difficulty,” defined as follows:
      • -
        operative vaginal delivery considered difficult by the operator (3 categories: easy, average, or difficult);
      • -
        and/or ≥2 vacuum device detachments (if vacuum device used);
      • -
        and/or need to apply a second instrument;
      • -
        and/or extraction duration of >10 minutes;
      • -
        and/or need for internal obstetrical maneuver to disengage the shoulders;
      • -
        and/or cesarean delivery for extraction failure.
      Secondary assessment criteria were all the criteria constituting the principal criterion taken individually, the existence of a third- or fourth-degree perineal tear, an umbilical artery pH of <7.0, and an Apgar score at 5 minutes’ life of <5.

      Instrumental extraction technique

      Prior to each attempt of operative vaginal delivery, the operator performed digital vaginal examination noting the fetal head station based on the numeric classification of the ACOG
      • Cunningham F.
      • MacDonald P.C.
      • Gant N.F.
      • et al.
      Conduct of normal labor and delivery.
      and in relation to the outlet (high, mid, or low). The presentation type was determined by digital vaginal examination, which was followed up by suprapubic ultrasound, as recommended by the French National College of Gynecologist and Obstetricians (CNGOF).
      • Vayssière C.
      • Beucher G.
      • Sentilhes L.
      • et al.
      Instrumental delivery: clinical practice guidelines from the French College of Gynecologists and Obstetricians.
      The type of instrument used was left to the discernment of the operator but was in most cases a vacuum extractor, except for deliveries <37 weeks or if there was a significant caput.
      Operative vaginal delivery was performed with the patient in the lithotomy position, bladder empty, with the fetal head engaged in the pelvis. Following the recommendations of the CNGOF, decisions to proceed with instrumental delivery when the head was in the high position in the pelvis were uncommon and in no case was extraction attempted on a fetal head that was not yet engaged.
      • Vayssière C.
      • Beucher G.
      • Sentilhes L.
      • et al.
      Instrumental delivery: clinical practice guidelines from the French College of Gynecologists and Obstetricians.
      Ultrasound measurement of the fetal head station was performed before instrumental delivery, just after having carried out suprapubic ultrasonography to determine the presentation type.
      • Chou M.R.
      • Kreiser D.
      • Taslimi M.M.
      • Druzin M.L.
      • El-Sayed Y.Y.
      Vaginal versus ultrasound examination of fetal occiput position during the second stage of labor.
      The operator performing ultrasound and extraction was a senior physician or an experienced obstetrician-gynecologist resident working with the approval of the senior obstetrician responsible for the delivery room. Both senior physicians and residents were trained in how to perform this measurement.

      Measuring method

      Transperineal ultrasound enabled linear measurement of fetal head station and was conducted during the resting phase following the method described by the team of Fouché et al.
      • Fouché C.J.
      • Simon E.G.
      • Potin J.
      • Perrotin F.
      Ultrasound in monitoring of the second stage of labor.
      The ultrasonograph was a portable machine (Voluson i; General Electric, Fairfield, CT). The abdominal probe protected by a sterile glove and covered with gel was applied horizontally to the perineal body, without intruding into the genital tract or pressing on tissues so as not to distort the structures being measured (Figure 1).
      Figure thumbnail gr1
      Figure 1Measurement method of perineum-to-skull ultrasound distance
      Kasbaoui et al. Predicting difficulty of operative vaginal delivery by ultrasound. Am J Obstet Gynecol 2017.
      The image obtained was a coronal view of the perineum and maternal pelvis enabling the external bony limit of the fetal skull to be visualized. The PSUD corresponded to the distance measured between the ultrasonographic probe and the closest part of the bony structures of the fetal skull (Figure 2).
      Figure thumbnail gr2
      Figure 2Ultrasound of perineum-to-skull measurement
      Kasbaoui et al. Predicting difficulty of operative vaginal delivery by ultrasound. Am J Obstet Gynecol 2017.
      Whenever possible, a second operator performed a second measurement without knowing the outcome of the first. In case of nonreassuring fetal heart rate and immediate extraction indicated, PSUD was omitted so as not to delay management of the delivery.
      The first operator, who performed the first ultrasound measurement, was the one who attempted the operative vaginal delivery.

      Statistical analysis

      To assess the generalizability of results to all patients eligible for instrumental vaginal delivery >34 weeks, maternal and obstetrical characteristics were firstly compared between patients who did and did not undergo measurement of the PSUD. Quantitative variables were presented using mean ± SD, then compared by means of Student t test. Gaussian distributions were assessed graphically and by using Shapiro-Wilk test. Qualitative variables were presented as numbers and percentages, and then compared between groups using Pearson χ2 test or Fisher exact test depending on total numbers. Secondly, ultrasound distances were compared for each key study criterion using Student t test.
      Analysis of the receiver operating characteristic (ROC) curve enabled assessment of PSUD performances and fetal head station for the diagnosis of difficult extraction. Performance indicators and thresholds for test measurements corresponding to Youden index maximizations were calculated. Areas under the curve for both tests were also assessed and then compared using a bootstrap resampling method.
      A multivariate logistic regression model enabled us to evaluate the relationship between PSUD and the occurrence of extraction difficulty by adjusting for confounding variables clinically recognized in the literature: nulliparity, type of cephalic presentation, and macrosomia. Results were presented as odds ratios (OR) with their 95% confidence intervals (CI). To ensure robustness of our findings, sensitivity analyses were performed redefining the composite outcome excluding each component by turns.
      Interobserver reproducibility of ultrasound measurement (measurements obtained by 2 different operators) was assessed graphically using Bland and Altman method and by calculating the intraclass correlation coefficient (ICC) and its 95% CI. Reproducibility was evaluated using the patients’ body mass index (BMI) categories, which was sought to be a potential confounding factor.
      The correlation between ultrasound measurement and digital vaginal examination was measured by calculating Spearman rank correlation coefficient. A P value < .05 was considered as significant. Analyses were performed using software (R statistics, Version 3.2.2, R Foundation for Statistical Computing, Vienna, Austria).

      Results

      Description of the study population

      Of the 8135 live births >34 weeks that took place during the enrollment period, 973 (13%) operative vaginal deliveries were performed, among which 314 (32%) were excluded because the measurement of the PSUD was not performed. Finally, 659 patients (8%) were eligible for the study on the grounds of the inclusion and exclusion criteria. A flowchart summarizes how the study population was selected (Figure 3).
      Figure thumbnail gr3
      Figure 3Flow chart of study population
      PSUD, perineum-to-skull ultrasound distance.
      Kasbaoui et al. Predicting difficulty of operative vaginal delivery by ultrasound. Am J Obstet Gynecol 2017.

      Comparison of groups with and without ultrasound

      The type of instrument used was comparable in the group of patients in whom PSUD was measured compared to the group in which it was not measured. Proportions were 553 (90%) and 289 (92%) for vacuum devices, 41(6%) and 16 (5%) for spatula, and 25 (4%) and 9 (5%) for forceps, respectively.
      Table 1 compares maternal and obstetrical characteristics in the groups where PSUD was and was not measured. In terms of biometric characteristics, there was no significant difference between groups except for the BMI, which was slightly higher in patients who underwent ultrasonography; this difference was not, however, clinically relevant. In terms of obstetrical characteristics, nonreassuring fetal heart rate was slightly more pronounced in the group of patients who did not undergo PSUD measurement (25% vs 17%, P < .001). This corresponded to the fact that PSUD measurement was omitted so as not to delay case management in the event of nonreassuring fetal status. Overall, both groups were comparable for potential confounding factors.
      Table 1Comparison of maternal and obstetrical characteristics between groups with and without perineum-to-skull ultrasound distance measurement
      Maternal and obstetrical characteristicsPSUD measured, N = 659PSUD not measured, N = 314P
      Maternal age, y, mean ± SD31.3 ± 5.1930.7 ± 5.55.148
      Body mass index, kg/m2, mean ± SD28.3 ± 5.5127.4 ± 5.24.018
      Nulliparous, N (%)550 (83)268 (85.3).512
      Gestational age, wk, mean ± SD40.1 ± 1.2640.0 ± 1.35.051
      Epidural anesthesia, N (%)591 (89)285 (90).648
      Posterior or transverse presentation, N (%)161 (24)65 (21).195
      Nonreassuring fetal heart rate, N (%)85 (17)77 (25)<.0001
      Birthweight >4000 g, N (%)34 (5.2)11 (3.5).327
      PSUD, perineum-to-skull ultrasound distance.
      Kasbaoui et al. Predicting difficulty of operative vaginal delivery by ultrasound. Am J Obstet Gynecol 2017.

      Description of the judgment criteria

      Of the 659 patients in whom PSUD was measured prior to instrumental extraction, there were 24 (3.6%) cases where operative delivery was considered difficult by the operator, 61 (9.3%) cases of ≥2 vacuum device detachments, 31 (4.7%) cases of first instrument failure, 68 (10.3%) cases of extraction lasting >10 minutes, and 11 (1.7%) cases of obstetrical maneuver for shoulder dystocia. There were 3 cesareans deliveries due to failed vaginal operative delivery. In total, 120 (18%) instrumental vaginal extractions met the composite criterion for a difficult extraction. Cesareans deliveries, need to apply a second instrument, and operative vaginal delivery considered difficult by the operator were the components of the composite criterion with higher overlap rate (respectively, 100%, 87%, and 83%).

      Univariate analysis of the principal judgement criterion and secondary criteria

      The mean PSUD was 42.2 ± 11.7 mm. Table 2 compares mean PSUD readings according to the different criteria of extraction difficulty and neonatal data. Extraction difficulty based on the principal composite criterion was significantly associated with a higher PSUD (46.0 vs 40.1 mm, P < .0001). This PSUD was also significantly higher in the event of every single component of the composite criterion.
      Table 2Comparison of mean perineum-to-skull ultrasound distance according to different criteria of extraction difficulty and neonatal data
      Extraction difficulty and neonatal criteriaMean PSUD, mmP
      Operative vaginal delivery considered difficult by operator

      N = 24
      51.1.0001
      Operative vaginal delivery considered easy by operator

      N = 633
      40.9
      Extraction duration of ≥10 min

      N = 68
      47.8<.0001
      Extraction duration <10 min

      N = 591
      40.4
      ≥2 Vacuum device detachments

      N = 61
      44.9.02
      ≤1 Vacuum device detachments

      N = 492
      40.8
      Need for second extraction instrument

      N = 31
      50.0<.0001
      No need for second extraction instrument

      N = 628
      40.8
      Shoulder dystocia

      N = 11
      49.0.04
      No shoulder dystocia

      N = 648
      41.1
      Extraction difficulty based on principal composite criterion

      N = 120
      46.0<.0001
      No extraction difficulty based on principal composite criterion

      N = 539
      40.1
      Arterial pH ≤7

      N = 9
      42.1.84
      Arterial pH >7

      N = 621
      41.3
      5-min Apgar ≤5

      N = 7
      39.0.65
      5-min Apgar >5

      N = 635
      41.2
      Presence of perineal lesions ≥3rd degree

      N = 47
      41.4.92
      Presence of perineal lesions <3rd degree

      N = 612
      41.2
      PSUD, perineum-to-skull ultrasound distance.
      Kasbaoui et al. Predicting difficulty of operative vaginal delivery by ultrasound. Am J Obstet Gynecol 2017.
      There was no statistically significant association between PSUD and neonatal outcomes.
      The ROC curve for predicting the occurrence of the composite criterion as a function of the PSUD is shown in Figure 4. The area under the curve was 0.63 (95% CI, 0.58–0.69; P < .01). Youden index maximization was attained at a PSUD threshold of 39 mm. At the 40-mm threshold, the detection rate for the occurrence of a difficult extraction based on the composite criterion was 73.3% for a specificity of 47.6%. Table 3 displays the sensitivities, specificities, positive and negative predictive values, and positive and negative likelihood ratios for different thresholds.
      Figure thumbnail gr4
      Figure 4Receiver operating characteristic curve for predicting occurrence of composite criterion as function of perineum-to-skull ultrasound distance
      AUC, area under curve.
      Kasbaoui et al. Predicting difficulty of operative vaginal delivery by ultrasound. Am J Obstet Gynecol 2017.
      Table 3Sensitivities, specificities, and positive and negative predictive values for different thresholds of perineum-to-skull ultrasound distance
      ThresholdsSensitivitySpecificityPositive predictive valueNegative predictive value
      40 mm73.347.623.788.9
      50 mm4078.028.786.4
      60 mm18.393.337.983.1
      Kasbaoui et al. Predicting difficulty of operative vaginal delivery by ultrasound. Am J Obstet Gynecol 2017.

      Multivariate analyses

      Multivariate analyses on predicting the occurrence of a difficult operative delivery based on the composite criterion were conducted by taking different PSUD threshold measurements: they are shown in Table 4, Table 5, Table 6 for thresholds of 40, 50, and 60 mm, respectively. PSUD measurement of ≥40 mm was significantly associated with the occurrence of a difficult extraction based on the composite criterion, after adjustment for parity, presentation type, and fetal macrosomia (OR, 2.38; 95% CI, 1.51–3.74; P = .0002). Thresholds of 50 and 60 mm were associated with the occurrence of a difficult extraction with the following respective OR: 2.16 (95% CI, 1.40–3.33; P = .0005) and 3.02 (95% CI, 1.68–5.43; P = .0002). In the sensitivity analyses the association between PSUD measurement and the occurrence of a difficult extraction remained significant and the strength of this association did not change notably.
      Table 4Multivariate analysis on predicting occurrence of difficult operative delivery based on composite criterion taking perineum-to-skull ultrasound distance threshold up to 40 mm
      Adjustment criteriaOdds ratio95% CIP
      PSUD >40 mm2.381.51–3.74.0002
      Nulliparous1.600.89–2.89.12
      Presentation type: posterior or transverse1.651.06–2.57.02
      Birthweight >4000 g2.741.30–5.79.008
      CI, confidence interval; PSUD, perineum-to-skull ultrasound distance.
      Kasbaoui et al. Predicting difficulty of operative vaginal delivery by ultrasound. Am J Obstet Gynecol 2017.
      Table 5Multivariate analysis on predicting occurrence of difficult operative delivery based on composite criterion taking perineum-to-skull ultrasound distance threshold up to 50 mm
      Adjustment criteriaOdds ratio95% CIP
      PSUD >50 mm2.1601.40–3.33.0005
      Nulliparous1.5400.86–2.77.15
      Presentation type: posterior or transverse1.6391.05–2.56.03
      Birthweight >4000 g2.9131.38–6.15.005
      CI, confidence interval; PSUD, perineum-to-skull ultrasound distance.
      Kasbaoui et al. Predicting difficulty of operative vaginal delivery by ultrasound. Am J Obstet Gynecol 2017.
      Table 6Multivariate analysis on predicting occurrence of difficult operative delivery based on composite criterion taking perineum-to-skull ultrasound distance threshold up to 60 mm
      Adjustment criteriaOdds ratio95% CIP
      PSUD >60 mm3.021.68–5.43.0002
      Nulliparous1.520.84–2.75.17
      Presentation type: posterior, or transverse1.861.19–2.88.006
      Birthweight >4000 g2.911.37–6.18.005
      CI, confidence interval; PSUD, perineum-to-skull ultrasound distance.
      Kasbaoui et al. Predicting difficulty of operative vaginal delivery by ultrasound. Am J Obstet Gynecol 2017.

      Analysis of reproducibility

      The ICC between the PSUD measured by the first operator and that measured by the second operator was 0.96 (95% CI, 0.95–0.97; P < .0001). Interobserver reproducibility remained high in patients with a BMI of between 30-35 (severe obesity) and >35 (morbid obesity), with an ICC of 0.97 (95% CI, 0.94–0.98; P < .0001) and 0.97 (95% CI, 0.94–0.99; P < .0001), respectively.

      Comparison of ultrasonography and digital vaginal examination

      Correspondence between PSUD measurements and digital vaginal examination data (expressed as high, mid, or low) is shown in Table 7. Figure 5 displays PSUD measurements in relation to digital vaginal examination data (expressed using the ACOG classification). There is a moderate but significant correlation between ultrasound measurement and clinical examination findings (r = 0.33, P < .01).
      Table 7Correspondences between perineum-to-skull ultrasound distance measurements and digital vaginal examination data
      Digital vaginal examinationHighMidLowP
      n (%)77 (11.7%)475 (72%)76 (11.5%)<.001
      Mean PSUD, mm ± SD50 ± 11.040 ± 11.330 ± 9.9
      PSUD, perineum-to-skull ultrasound distance.
      Kasbaoui et al. Predicting difficulty of operative vaginal delivery by ultrasound. Am J Obstet Gynecol 2017.
      Figure thumbnail gr5
      Figure 5Perineum-to-skull ultrasound distance measurements in relation to digital vaginal examination
      Perineum-to-skull ultrasound distance (PSUD) measurements in relation to digital vaginal examination.
      ACOG, American Congress of Obstetricians and Gynecologists.
      Kasbaoui et al. Predicting difficulty of operative vaginal delivery by ultrasound. Am J Obstet Gynecol 2017.
      The ROC curve for predicting the occurrence of the composite criterion in relation to fetal head station based on the ACOG classification is displayed in Figure 6. The area under the curve was 0.55 (95% CI, 0.50–0.60; P < .01), which is significantly less than the area under the ROC curve for the PSUD (P = .036).
      Figure thumbnail gr6
      Figure 6Receiver operating characteristic curve for predicting occurrence of composite criterion in relation to fetal head station assessed by digital examination
      AUC, area under curve.
      Kasbaoui et al. Predicting difficulty of operative vaginal delivery by ultrasound. Am J Obstet Gynecol 2017.

      Comment

      Our study elicits an association between the occurrence of a difficult extraction and PSUD measurement. The 40-mm PSUD threshold is the threshold that optimizes sensitivity and specificity. Our study found good interobserver reproducibility for this measurement. Lastly, digital vaginal examination findings are also associated with the risk of a difficult extraction, but prediction is less reliable than with ultrasound.

      Strengths and weaknesses of the study

      To our knowledge, this is the largest study investigating the utility of linear ultrasound measurement. All the data were collected prospectively according to items defined specifically for the need of this study. This study is representative of the population using a level II maternity unit and the extraction rate in our population is similar to the average rate in France, which is 12%.
      There is a potential selection bias since one third of the eligible population did not undergo ultrasound even though these patients met the enrollment criteria. Nevertheless, the PSUD measured and not measured groups were comparable for potential confounding factors. In addition, PSUD measurement was not performed blind; this, however, had a limited influence on the extraction method since ultrasound was performed after deciding to proceed with instrumental extraction. Lastly, the instrumental vaginal extraction failure rate is too low to allow statistical analyses, although this reflects a cautious use of instrumental extraction.

      Interpretation

      Digital vaginal examination allows assessment of the fetal position and fetal head station. The utility of digital vaginal examination is, however, limited since there is a risk of error concerning the exact station of the presentation.
      • Dupuis O.
      • Gaucherand P.
      • Cucherat M.
      • et al.
      Birth simulator: reliability of transvaginal assessment of fetal head station as defined by the American College of Obstetricians and Gynecologists classification.
      Several studies evaluated the correlation between ultrasound data and fetal head station. Our study found that head station as assessed by digital examination was moderately but significantly correlated with ultrasonographic measurement. This was also the conclusion of the study by Tutschek et al,
      • Tutschek B.
      • Torkildsen E.A.
      • Eggebø T.M.
      Comparison between ultrasound parameters and clinical examination to assess fetal head station in labor.
      where the correlation is qualified as moderate, with a correlation coefficient of 0.52. The study by Maticot-Baptista et al
      • Maticot-Baptista D.
      • Ramanah R.
      • Collin A.
      • Martin A.
      • Maillet R.
      • Riethmuller D.
      Ultrasound in the diagnosis of fetal head engagement. A preliminary French prospective study.
      found that a PSUD >60 mm was consistent with a nonengaged presentation and the study from Rivaux et al
      • Rivaux G.
      • Dedet B.
      • Delarue E.
      • Depret S.
      • Closset E.
      • Deruelle P.
      The diagnosis of fetal head engagement: transperineal ultrasound, a new useful tool [in French]?.
      found that 66.4 mm corresponded with nonengaged presentations and 56.2 mm with high outlet presentations, which is consistent with our findings.
      An article by Fouché et al
      • Fouché C.J.
      • Simon E.G.
      • Potin J.
      • Perrotin F.
      Ultrasound in monitoring of the second stage of labor.
      describes different methods for measuring the level of fetal head engagement by ultrasound: abdominal, translabial, and modified translabial approaches. These methods are subdivided into linear methods, such as the transperineal ultrasound used in our study, and angular methods.
      • Simon E.-G.
      • Fouché C.-J.
      • Perrotin F.
      How I do… ultrasound in the diagnosis of fetal head engagement.
      Linear measurement is a straightforward technique which is well correlated with clinical examination findings
      • Tutschek B.
      • Torkildsen E.A.
      • Eggebø T.M.
      Comparison between ultrasound parameters and clinical examination to assess fetal head station in labor.
      and enables difficulties linked to the presence of a subcutaneous hematoma to be overcome.
      • Simon E.-G.
      • Fouché C.-J.
      • Perrotin F.
      How I do… ultrasound in the diagnosis of fetal head engagement.
      In respect of angular methods, the study by Bultez et al
      • Bultez T.
      • Quibel T.
      • Bouhanna P.
      • Popowski T.
      • Resche-Rigon M.
      • Rozenberg P.
      Angle of fetal head progression measured using transperineal ultrasound as a predictive factor of vacuum extraction failure.
      found that an angle of progression of <145.5 degrees was predictive of extraction failure. Conversely, according to Kalache et al,
      • Kalache K.D.
      • Dückelmann A.M.
      • Michaelis S.
      • Lange J.
      • Cichon G.
      • Dudenhausen J.W.
      Transperineal ultrasound imaging in prolonged second stage of labor with occipitoanterior presenting fetuses: how well does the ‘angle of progression’ predict the mode of delivery?.
      an angle of progression of >120 degrees was predictive of vaginal delivery. Angular methods are more difficult to apply because they employ virtual markers and take time to be calculated. The linear method based on the PSUD is attractive since it particularly simple and easy to execute.
      • Maticot-Baptista D.
      • Ramanah R.
      • Collin A.
      • Martin A.
      • Maillet R.
      • Riethmuller D.
      Ultrasound in the diagnosis of fetal head engagement. A preliminary French prospective study.
      • Simon E.-G.
      • Arthuis C.-J.
      • Perrotin F.
      Engagement of fetal head: what have we learnt from ultrasound [in French]?.
      Indeed, only a short theoretical training and 1 or 2 real cases are necessary to acquire this skill. Since ultrasound is readily available in the delivery room, the use of PSUD measurement seems to be easy to implement in current practice.
      Simplicity is potentially also a gauge of reproducibility and our data for PSUD measurement show excellent interobserver reproducibility as a previous study by Dietz and Lanzarone
      • Dietz H.P.
      • Lanzarone V.
      Measuring engagement of the fetal head: validity and reproducibility of a new ultrasound technique.
      in 90 patients. It was our hypothesis that reproducibility might be impaired by patient-based variations in the thickness of soft tissues, but stratification over different BMI levels shows that irrespective of the patient’s weight, measurement is patently reproducible.
      Some studies evaluating linear ultrasound measurements have proposed prognostic thresholds. Eggebø et al
      • Eggebø T.M.
      • Gjessing L.K.
      • Romundstad P.
      • et al.
      Prediction of labor and delivery by transperineal ultrasound in pregnancies with prelabor rupture of membranes at term.
      determined that there was a 45-mm threshold below which vaginal delivery was predictable. The study by Magnard et al
      • Magnard C.
      • Perrot M.
      • Fanget C.
      • Paviot-Trombert B.
      • Raia-Barjat T.
      • Chauleur C.
      Extraction instrumentale sur une hauteur de présentation supérieur à 55 mm à l’échographie transpérinéale.
      reported a significantly higher extraction failure rate in patients whose measurement was >55 mm or 45 mm and Tabard et al
      • Tabard F.
      • Feyeux C.
      • Sagot P.
      • et al.
      Correlation between the perineal-to-skull measurement by transperineal ultrasound, failure of vaginal operative delivery and maternal-fetal morbidity.
      came out with a threshold of 50 mm.
      Finally, regardless the ultrasound method used to measure the level of fetal head station, the assessment during pushing phase rather than during resting phase is of great interest. A recent study evaluating intrapartum translabial ultrasound with pushing suggests that it is a best predictor than ultrasound in rest.
      • Antonio Sainz J.
      • Borrero C.
      • Aquise A.
      • García-Mejido J.A.
      • Gutierrez L.
      • Fernández-Palacín A.
      Intrapartum translabial ultrasound with pushing used to predict the difficulty in vacuum-assisted delivery of fetuses in non-occiput posterior position.
      It is a promising trail and further studies are necessary.
      In sum, the higher the PSUD measurement, the greater the risk of a difficult or even failed extraction. In our study, the risk of an extraction difficulty was 2 and 3 times greater for thresholds of 40 and 60 mm, respectively. As the results of our multivariate analyses show, it is important to take into account other predictive factors such as parity, presentation lie, and expected fetal weight. Integration of ultrasound data and all these parameters should allow upfront assessment of the potential difficulty of a vaginal operative delivery and the possible option of cesarean delivery to be chosen depending on the operator’s experience. It is to be noted that the strength of the association between the PSUD measurement and a difficult extraction is about the same as the association between a macrosomic fetus and a difficult extraction, and more important than the association between nulliparity or nonanterior presentation and a difficult extraction.
      In conclusion, measurement of the PSUD is a reproducible and predictive index of the difficulty of instrumental extraction. Ultrasound is a useful supplementary tool to the usual clinical findings.

      Acknowledgment

      We thank all the patients and physicians who took part in this study.

      References

      1. Zettlin J, Mohangoo A, Delnord M. Health and care of pregnant women and babies in Europe in 2010. In: European Perinatal Health Report, ed. Belgium: European Perinatal Health Report. 2012:77-80.

      2. Blondel B, Kermarec M. Les naissances en 2010 et leur évolution depuis 2003. In: INSERM, ed. Paris, France: Enquête Nationale Périnatale 2010. 2011.

        • Vayssière C.
        • Beucher G.
        • Sentilhes L.
        • et al.
        Instrumental delivery: clinical practice guidelines from the French College of Gynecologists and Obstetricians.
        Eur J Obstet Gynecol Reprod Biol. 2011; 159: 43-48
        • Knight D.
        • Newnham J.P.
        • McKenna M.
        • Evans S.
        A comparison of abdominal and vaginal examinations for the diagnosis of engagement of the fetal head.
        Aust N Z J Obstet Gynaecol. 1993; 33: 154-158
        • Dupuis O.
        • Gaucherand P.
        • Cucherat M.
        • et al.
        Birth simulator: reliability of transvaginal assessment of fetal head station as defined by the American College of Obstetricians and Gynecologists classification.
        Am J Obstet Gynecol. 2005; 192: 868-874
        • Cunningham F.
        • MacDonald P.C.
        • Gant N.F.
        • et al.
        Conduct of normal labor and delivery.
        in: Williams obstetrics. Appleton Lange, Stamford (CT)1997
        • Bultez T.
        • Quibel T.
        • Bouhanna P.
        • Popowski T.
        • Resche-Rigon M.
        • Rozenberg P.
        Angle of fetal head progression measured using transperineal ultrasound as a predictive factor of vacuum extraction failure.
        Ultrasound Obstet Gynecol. 2016; 48: 86-91
        • Fouché C.J.
        • Simon E.G.
        • Potin J.
        • Perrotin F.
        Ultrasound in monitoring of the second stage of labor.
        Gynecol Obstet Fertil. 2012; 40 ([in French]): 658-665
        • Eggebø T.M.
        • Gjessing L.K.
        • Romundstad P.
        • et al.
        Prediction of labor and delivery by transperineal ultrasound in pregnancies with prelabor rupture of membranes at term.
        Ultrasound Obstet Gynecol. 2006; 27: 387-391
        • Maticot-Baptista D.
        • Ramanah R.
        • Collin A.
        • Martin A.
        • Maillet R.
        • Riethmuller D.
        Ultrasound in the diagnosis of fetal head engagement. A preliminary French prospective study.
        J Gynecol Obstet Biol Reprod. 2009; 38: 474-480
        • Magnard C.
        • Perrot M.
        • Fanget C.
        • Paviot-Trombert B.
        • Raia-Barjat T.
        • Chauleur C.
        Extraction instrumentale sur une hauteur de présentation supérieur à 55 mm à l’échographie transpérinéale.
        Gynecol Obstet Fertil. 2016; 44 ([in French]): 82-87
        • Chou M.R.
        • Kreiser D.
        • Taslimi M.M.
        • Druzin M.L.
        • El-Sayed Y.Y.
        Vaginal versus ultrasound examination of fetal occiput position during the second stage of labor.
        Am J Obstet Gynecol. 2004; 191: 521-524
        • Tutschek B.
        • Torkildsen E.A.
        • Eggebø T.M.
        Comparison between ultrasound parameters and clinical examination to assess fetal head station in labor.
        Ultrasound Obstet Gynecol. 2013; 41: 425-429
        • Rivaux G.
        • Dedet B.
        • Delarue E.
        • Depret S.
        • Closset E.
        • Deruelle P.
        The diagnosis of fetal head engagement: transperineal ultrasound, a new useful tool [in French]?.
        Gynecol Obstet Fertil. 2012; 40: 148-152
        • Simon E.-G.
        • Fouché C.-J.
        • Perrotin F.
        How I do… ultrasound in the diagnosis of fetal head engagement.
        Gynecol Obstet Fertil. 2012; 40 ([in French]): 625-627
        • Kalache K.D.
        • Dückelmann A.M.
        • Michaelis S.
        • Lange J.
        • Cichon G.
        • Dudenhausen J.W.
        Transperineal ultrasound imaging in prolonged second stage of labor with occipitoanterior presenting fetuses: how well does the ‘angle of progression’ predict the mode of delivery?.
        Ultrasound Obstet Gynecol. 2009; 33: 326-330
        • Simon E.-G.
        • Arthuis C.-J.
        • Perrotin F.
        Engagement of fetal head: what have we learnt from ultrasound [in French]?.
        Gynecol Obstet Fertil. 2014; 42: 375-377
        • Dietz H.P.
        • Lanzarone V.
        Measuring engagement of the fetal head: validity and reproducibility of a new ultrasound technique.
        Ultrasound Obstet Gynecol. 2005; 25: 165-168
        • Tabard F.
        • Feyeux C.
        • Sagot P.
        • et al.
        Correlation between the perineal-to-skull measurement by transperineal ultrasound, failure of vaginal operative delivery and maternal-fetal morbidity.
        J Gynecol Obstet Biol Reprod. 2013; 42: 541-549
        • Antonio Sainz J.
        • Borrero C.
        • Aquise A.
        • García-Mejido J.A.
        • Gutierrez L.
        • Fernández-Palacín A.
        Intrapartum translabial ultrasound with pushing used to predict the difficulty in vacuum-assisted delivery of fetuses in non-occiput posterior position.
        J Matern Fetal Neonatal Med. 2016; 29: 3400-3405

      Linked Article

      • Comment on: Predicting the difficulty of operative vaginal delivery by ultrasound measurements of the fetal head station
        American Journal of Obstetrics & GynecologyVol. 218Issue 1
        • Preview
          I read with interest the study by Kasbaoui et al,1 who investigated the clinical usefulness of measuring the perineum-to-skull ultrasound distance to predict the difficulty of operative vaginal delivery. However, I cannot agree with the conclusion reached. To perform operative vaginal delivery, I believe that the position of the largest circumference of the fetal head is more important than that of the lowest part of the fetal head. We sometimes have encountered cases in which the position of the largest circumference was higher than the pelvic inlet, whereas the lowest part was descending because of molding of the fetal head, particularly in cases of occiput-posterior presentation.
        • Full-Text
        • PDF
      • Reply
        American Journal of Obstetrics & GynecologyVol. 218Issue 1
        • Preview
          We thank Dr Suzuki for the comment about our article that showed that transperineal ultrasound measurement of the distance between the leading part of the fetal skull and the perineum was a useful supplementary tool to predict difficult operative vaginal delivery.1
        • Full-Text
        • PDF
      • Comment on: Predicting the difficulty of operative vaginal delivery by ultrasound measurement of fetal head station
        American Journal of Obstetrics & GynecologyVol. 217Issue 3
        • Preview
          We read with interest the study by Kasbaoui et al,1 who investigated whether measurement of the perineum-to-skull ultrasound distance was predictive of a difficult operative vaginal delivery (OVD) using a prospective cohort study. The main finding highlights that measurement of ≥40 mm is a reproducible and predictive index of the difficulty of OVD. We congratulate the authors on their efforts to define an optimum threshold distance to predict a difficult OVD.
        • Full-Text
        • PDF