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Levator hiatus dimensions in late pregnancy and the process of labor: a 3- and 4-dimensional transperineal ultrasound study

Published:February 24, 2014DOI:https://doi.org/10.1016/j.ajog.2014.02.021

      Objective

      The objectives of the investigation were to study the association between levator hiatus dimensions in late pregnancy and both the length of second stage of labor and also the delivery mode in women delivering their first child.

      Study Design

      In this cohort study, 231 nulliparous women were examined with 3- and 4-dimensional transperineal ultrasonography at 37 weeks of gestation. The anteroposterior, transverse diameter, and the area of levator hiatus were measured at rest, during levator ani muscle contraction, and during Valsalva maneuver. The second stage of labor was divided into passive and active second stage and delivery modes into normal vaginal or instrumental deliveries. Spearman correlation coefficient, independent-sample t test, and standard logistic regression were used for analysis.

      Results

      Larger levator hiatus dimensions at rest and during contraction at 37 weeks of gestation correlated with a shorter duration of the active second stage in women with normal vaginal delivery (Spearman correlation coefficient, –0.13 to –0.35, P ≤ .08). Women having normal vaginal deliveries had significantly larger transverse diameter at rest, during contraction, and during Valsalva maneuver compared with women having instrumental deliveries (mean difference, 0.29; 95% confidence interval (CI), 0.16–0.41; mean difference, 0.33; 95% CI, 0.21–0.44 and mean difference, 0.24; 95% CI, 0.06–0.42; P < .05). The same was true for the levator hiatus area at rest and during contraction (mean difference, 1.22; 95% CI, 0.37–2.07 and mean difference, 0.84; 95% CI, 0.22–1.46; P < .01). These estimates were unchanged by adjustments in the logistic regression analysis.

      Conclusion

      Larger levator hiatus dimensions in late pregnancy had a significant association with a shorter active second stage of labor and normal vaginal delivery.

      Key words

      Lack of progression of labor can be caused by many factors such as insufficient uterine contractions, size and presentation of the fetus, and the size of the pelvis.
      American College of Obstetricians and Gynecologists
      Dystocia and augmentation of labor. ACOG Practice bulletin no. 49.
      When passing the birth canal, the fetus meets resistance not only from the bony pelvis but also from soft tissue, in particular the levator ani muscle.
      • Parente M.P.
      • Natal Jorge R.M.
      • Mascarenhas T.
      • Fernandes A.A.
      • Silva-Filho A.L.
      Computational modeling approach to study the effects of fetal head flexion during vaginal delivery.
      The most medial part of the levator ani muscle, the symphysis, and inferior ramus pubis border the area of levator hiatus. Hence, if the levator hiatus is of limited size, this may have the potential to negatively influence the progress of labor, especially the second stage. This might eventually cause fetal distress and exhausted mothers, in turn necessitating instrumental intervention for delivery.
      Complicated deliveries can cause adverse outcomes for the child and also serious injuries to the women's pelvic floor
      • Dietz H.P.
      Clinical consequences of levator trauma.
      • Delancey J.O.
      • Kearney R.
      • Chou Q.
      • Speights S.
      • Binno S.
      The appearance of levator ani muscle abnormalities in magnetic resonance images after vaginal delivery.
      • Allen R.E.
      • Hosker G.L.
      • Smith A.R.
      • Warrell D.W.
      Pelvic floor damage and childbirth: a neurophysiological study.
      and anal sphincter muscle,
      • Hehir M.P.
      • O'Connor H.D.
      • Higgins S.
      • et al.
      Obstetric anal sphincter injury, risk factors and method of delivery—an 8-year analysis across two tertiary referral centers.
      • Gurol-Urganci I.
      • Cromwell D.A.
      • Edozien L.C.
      • et al.
      Third- and fourth-degree perineal tears among primiparous women in England between 2000 and 2012: time trends and risk factors.
      accompanied by loss of function and quality of life.
      • Ashton-Miller J.A.
      • Delancey J.O.
      Functional anatomy of the female pelvic floor.
      To date, reliable identification of women at risk is not available, and there is a need for an increased understanding of the mechanisms that may lead to difficult vaginal or abdominal operative deliveries. Only a few studies have assessed the influence of levator ani muscle anatomy in late pregnancy on delivery outcome, and results were inconclusive.
      • Lanzarone V.
      • Dietz H.P.
      Three-dimensional ultrasound imaging of the levator hiatus in late pregnancy and associations with delivery outcomes.
      • Toozs-Hobson P.
      • Balmforth J.
      • Cardozo L.
      • Khullar V.
      • Athanasiou S.
      The effect of mode of delivery on pelvic floor functional anatomy.
      Thus, the aim of the present study was to study the associations between levator hiatus dimensions measured with 3- and 4-dimensional transperineal ultrasonography at 37 weeks of gestation and both the length of the passive and active second stage of labor and also the delivery mode in 231 women delivering their first child.

      Materials and Methods

      In this cohort study, 300 pregnant nulliparous women were recruited between December 2009 and April 2011 at Akershus University Hospital (Norway). During this period, all nulliparous women attending the hospital for routine prenatal ultrasound examinations at 18 weeks of gestation were invited to participate.
      Inclusion criteria were a singleton pregnancy and being able to understand one of the Scandinavian languages. Women with a previous pregnancy of more than 16 weeks of gestation were not included. In this study, ongoing exclusion criteria were stillbirth, serious illness of the mother or child, missing delivery data, prelabor cesarean section, and cesarean section before full cervical dilatation. The Regional Ethics Committee (REK Sør-Øst D 2009/170) and the Norwegian Social Science Data Service (2799026) approved the study, and all women gave informed written consent to participate.
      The women were examined at 22 and 37 weeks of gestation using 3- and 4-dimensional transperineal ultrasonography. The ultrasound examination of levator ani muscle was performed by 2 trained investigators, using a GE Voluson E8 system (GE Medical Systems, Zipf, Austria) with a 4-8 MHz curved array volume transducer (RAB4-8l/obstetric). The ultrasound volumes were acquired with the women in the lithotomy position, with empty bladder at rest, during maximum levator ani muscle contraction, and maximum Valsalva maneuver.
      • Siafarikas F.
      • Staer-Jensen J.
      • Braekken I.H.
      • Bo K.
      • Engh M.E.
      Learning process for performing and analyzing 3D/4D transperineal ultrasound imaging and interobserver reliability study.
      Care was taken to avoid cocontraction of the most medial part of the levator ani muscle during the Valsalva maneuver.
      • Orno A.K.
      • Dietz H.P.
      Levator co-activation is a significant confounder of pelvic organ descent on Valsalva maneuver.
      The ultrasound images were stored offline using anonymous code numbers and were analyzed using 4-dimensional software (version 10.0; GE Healthcare, Fairfield, CT). Render mode
      • Dietz H.P.
      • Wong V.
      • Shek K.L.
      A simplified method for determining hiatal biometry.
      around the plane of minimal hiatal dimensions was used. Measurements of the levator hiatus were of anteroposterior and transverse diameter and the levator hiatus area itself (Figure).
      • Dietz H.P.
      • Shek C.
      • Clarke B.
      Biometry of the pubovisceral muscle and levator hiatus by three-dimensional pelvic floor ultrasound.
      • Braekken I.H.
      • Majida M.
      • Ellstrom-Engh M.
      • Dietz H.P.
      • Umek W.
      • Bo K.
      Test-retest and intra-observer repeatability of two-, three- and four-dimensional perineal ultrasound of pelvic floor muscle anatomy and function.
      • Majida M.
      • Braekken I.H.
      • Umek W.
      • Bo K.
      • Saltyte Benth J.
      • Ellstrom Engh M.
      Interobserver repeatability of three- and four-dimensional transperineal ultrasound assessment of pelvic floor muscle anatomy and function.
      Volume rendering is a technique used to display a 2-dimensional projection of a 3-dimensional structure. In this case, ultrasonography of the levator ani muscle in the midsagittal, the axial, and the coronal 2-dimensional planes produce a semitransparent rendered representation of the pelvic floor.
      • Stær-Jensen J.
      • Siafarikas F.
      • Hilde G.
      • Bø K.
      • Ellstrom Engh M.
      Ultrasonographic evaluation of pelvic organ support during pregnancy.
      It provides a semitransparent representation of all gray-scale pixels within a render box, which contains the plane of minimal hiatal dimension also the region of interest.
      • Dietz H.P.
      • Wong V.
      • Shek K.L.
      A simplified method for determining hiatal biometry.
      The render box was approximately 1-2 cm thick.
      • Dietz H.P.
      • Wong V.
      • Shek K.L.
      A simplified method for determining hiatal biometry.
      Hence, it covers the entire craniocaudal space by the non-Euclidean shape of the levator hiatus.
      • Dietz H.P.
      • Wong V.
      • Shek K.L.
      A simplified method for determining hiatal biometry.
      • Kruger J.A.
      • Heap S.W.
      • Murphy B.A.
      • Dietz H.P.
      How best to measure the levator hiatus: evidence for the non-Euclidean nature of the ‘plane of minimal dimensions’.
      This results in an axial image that corresponds to an observation of the patient's pelvic floor as seen from below.
      • Dietz H.P.
      Pelvic floor ultrasound: a review.
      Figure thumbnail gr1
      FigureLevator hiatus dimensions from the same patient acquired during the ultrasound examination at 37 weeks of gestation
      Illustration of the different appearances of the levator hiatus dimensions during the 3 maneuvers: A, at rest, B, during maximum levator ani muscle contraction, and C, during the Valsalva maneuver. Broken dotted lines indicate the anteroposterior diameter of the levator hiatus, measured as the diameter between the inferior rim of the symphysis pubis and the medial part of the levator ani muscle. Solid lines indicate the transverse diameter of the levator hiatus, measured as the widest diameter of the levator hiatus from right to left, perpendicular to the anteroposterior diameter. Broken lines indicate the levator hiatus area, measured as the area bordered by the medial part of the levator ani muscle, symphysis pubis, and inferior pubic ramus. The X indicates the levator ani muscle insertion into the pubic bone.
      Siafarikas. Levator hiatus dimensions and the process of labor. Am J Obstet Gynecol 2014.
      In the present study, 4 trained investigators analyzed the images. Intraclass correlation coefficient (ICC) values for interobserver reliability of the 4 investigators were good to excellent (ICC >0.80) for all measurements, except for the transverse diameter of the levator hiatus at rest and during Valsalva maneuver (ICC >0.62).
      • Stær-Jensen J.
      • Siafarikas F.
      • Hilde G.
      • Bø K.
      • Ellstrom Engh M.
      Ultrasonographic evaluation of pelvic organ support during pregnancy.
      The assessors were blinded to the women's demographic data and obstetric history, information that was retrieved from the hospital's electronic birth records.
      Obstetric data such as induction of labor were coded yes or no. Epidural analgesia (yes/no) was given as continuous infusion with the possibility of top-ups. The labor augmentation (yes/no) included amniotomy and oxytocin administration and breast stimulation.
      The second stage of labor was divided into passive and active second stage. Passive second stage was defined as the interval between full cervical dilatation and the commencement of active pushing.

      National Institute for Health and Care Excellence (NICE). Intrapartum care. Care of healthy women and their babies during childbirth. NICE clinical guideline 55. 2007. Available at: http://guidance.nice.org.uk/cg55. Accessed Oct. 1, 2013.

      Active second stage was defined as the time of active pushing until the delivery of the infant.

      National Institute for Health and Care Excellence (NICE). Intrapartum care. Care of healthy women and their babies during childbirth. NICE clinical guideline 55. 2007. Available at: http://guidance.nice.org.uk/cg55. Accessed Oct. 1, 2013.

      To determine the impact of levator hiatus dimensions on delivery mode, women were stratified into having a normal or an instrumental delivery. The instrumental delivery group included all women having vacuum, forceps, or cesarean section after full cervical dilatation.
      Statistical analysis was performed using SPSS version 20 (SPSS Inc, Chicago, IL). Demographic and obstetric data were presented as frequencies with percentages, means with SDs, or medians with ranges for normally or nonnormally distributed data. The correlation between levator hiatus dimensions and active and passive second stage of labor was estimated using the Spearman correlation coefficient. Differences in levator hiatus measurements in women with normal vaginal and instrumental deliveries were given as the mean differences with 95% confidence intervals (CIs) and were analyzed using an independent-sample Student t test.
      To control for possible covariates when analyzing the influence of levator hiatus measurements on delivery mode, a standard logistic regression analysis was used. The selection of covariates was based on existing literature and clinical reasoning. Covariates found to have a significant association (P < .05) with delivery mode or ultrasound measurements were taken into the regression model. Logistic regression analysis was performed for each levator hiatus measurement individually along with the covariates. Crude and adjusted odds ratio (OR) with 95% CI were reported. Values of P < .05 were considered significant.

      Results

      Of the 300 women recruited, 7 women did not attend the ultrasound examination at 37 weeks of gestation, 16 women were not examined owing to delivery before ultrasound examination, and 3 women were excluded because of stillbirths. Seventeen women had a prelabor cesarean section, 16 women had a cesarean section before full cervical dilatation, and delivery data were missing for 10 women. This left 231 women in the study.
      The age, prepregnancy body mass index, fetal birthweight, and delivery mode of the 300 women recruited was comparable with the total population of pregnant nulliparous women scheduled to deliver at Akershus University Hospital during the inclusion period (n = 2547).
      Table 1 presents demographic and obstetric data for the participants. One hundred eighty- four women (79.7%) had a normal and 47 (20.3%) had an instrumental delivery (38 vacuum, 2 forceps, 2 vacuum and forceps, and 5 cesarean section after full cervical dilatation). Indications for instrumental intervention were failure to progress (n = 27), fetal distress (n = 18), abnormal fetal presentation (n = 1), and exhausted mother (n = 1). A comparison of women who had normal vaginal deliveries with those who had instrumental deliveries showed significantly lower fetal birthweight and shorter duration of the second stage of labor. Labor augmentation and epidural analgesia were more frequently used in instrumental deliveries than in normal vaginal deliveries (Table 1).
      Table 1Demographic and obstetric data of the study population
      DemographicAll (n = 231)Normal vaginal delivery (n = 184)Instrumental delivery (n = 47)P value
      Age, y
      Values are given as mean with SD
      29.3 (4.1)29.3 (4.3)29.4 (3.6).84
      Prepregnancy body mass index, kg/m2
      Values are given as mean with SD
      23.9 (4.0)23.9 (4.1)23.8 (3.8).96
      Weeks of gestation at ultrasound examination
      Values are given as mean with SD
      36.8 (0.7)36.8 (0.7)36.8 (0.7).81
      Total gestational length, d
      Values are given as mean with SD
      282.5 (8.4)282.0 (8.4)284.6 (8.0).051
      Fetal birthweight, g
      Values are given as mean with SD
      3527.7 (444.0)3478.4 (434.5)3720.8 (431.7).001
      Length of passive second stage, min
      Median with range; or percentages.
      16.0 (0–325.0)15.0 (0–180.0)40.0 (0–325.0)< .01
      Length of active second stage, min
      Median with range; or percentages.
      34.0 (6.0–107.0)32.0 (6.0–102.0)51.0 (8.0–107.0)< .001
      Induction of labor16.0%15.8%17.0%.83
      Epidural analgesia41.6%35.9%63.8%.001
      Labor augmentation58.9%52.2%85.1%< .001
      Instrumental delivery was all women having vacuum, forceps, or cesarean section after full cervical dilatation. Missing data for duration of passive second stage in normal vaginal delivery group, n = 2. Duration of active second stage in normal vaginal delivery group, n = 1.
      Siafarikas. Levator hiatus dimensions and the process of labor. Am J Obstet Gynecol 2014.
      a Values are given as mean with SD
      b Median with range; or percentages.
      No correlation between levator hiatus dimensions and the length of the passive second stage of labor was found in the 2 delivery groups (data not shown). For women with normal vaginal delivery, there was a significant weak to moderate inverse correlation between anteroposterior diameter and levator hiatus area at rest and all levator hiatus dimensions during contraction, and the length of active second stage. In deliveries with instrumental intervention, no significant correlation between levator hiatus dimensions and length of active second stage was found (Table 2).
      Table 2Levator hiatus dimensions and duration of active second stage
      VariableLHap

      R
      LHrl

      R
      LHarea

      R
      LHap

      C
      LHrl

      C
      LHarea

      C
      LHap

      V
      LHrl

      V
      LHarea

      V
      Normal vaginal delivery (n = 183)r

      P value
      –0.23

      < .005
      –0.13

      .08
      –0.29

      < .001
      –0.29

      < .001
      –0.21

      < .05
      –0.35

      < .001
      –0.05

      .52
      –0.10

      .17
      –0.09

      .24
      Instrumental delivery (n = 47)r

      P value
      –0.18

      .23
      –0.13

      .38
      –0.11

      .47
      –0.06

      .97
      –0.05

      .73
      –0.10

      .51
      –0.12

      .43
      –0.16

      .28
      –0.16

      .30
      Levator hiatus dimensions measured at 37 weeks of gestation. Missing data for duration of active second stage in normal vaginal delivery group, n = 1.
      ap, anteroposterior diameter; C, contraction; LH, levator hiatus; rl, transverse diameter; r, Spearman correlation coefficient; R, rest; V, Valsalva maneuver.
      Siafarikas. Levator hiatus dimensions and the process of labor. Am J Obstet Gynecol 2014.
      Women having normal vaginal deliveries had significantly wider transverse diameter of levator hiatus at rest, during contraction, and during the Valsalva maneuver compared with women who required instrumental interventions. The same was true for levator hiatus area at rest and during contraction. The lengths of the anteroposterior diameter of the levator hiatus did not differ between delivery groups (Table 3).
      Table 3Levator hiatus dimensions according to delivery mode
      VariableNormal vaginal delivery (n = 184)Instrumental delivery (n = 47)Mean differenceP value
      LHapR, cm5.13 (0.69)5.05 (0.63)0.08 (–0.13 to 0.30).45
      LHrlR, cm3.82 (0.38)3.53 (0.35)0.29 (0.16–0.41)< .001
      LHareaR, cm213.92 (2.74)12.70 (2.16)1.22 (0.37–2.07)< .01
      LHapC, cm4.15 (0.59)4.15 (0.58)0.00 (–0.19 to 0.19).99
      LHrlC, cm3.53 (0.35)3.20 (0.36)0.33 (0.21–0.44)< .001
      LHareaC, cm210.61 (1.96)9.77 (1.77)0.84 (0.22–1.46)< .01
      LHapV, cm5.90 (1.07)5.84 (1.12)0.06 (–0.29 to 0.42).71
      LHrlV, cm4.21 (0.59)3.97 (0.51)0.24 (0.06–0.42)< .05
      LHareaV, cm218.91 (6.20)17.74 (5.44)1.17 (–0.65 to 3.00).24
      Levator hiatus dimensions measured at 37 weeks of gestation. Values within groups are given as mean and SD and between groups as mean differences with 95% confidence interval.
      ap, anteroposterior diameter; C, contraction; LH, levator hiatus; R, rest; rl, transverse; V, Valsalva maneuver.
      Siafarikas. Levator hiatus dimensions and the process of labor. Am J Obstet Gynecol 2014.
      Results from the logistic regression analysis are presented in Table 4. Each line in the table refers to a unique logistic regression analysis containing no other levator hiatus dimension other than the measure in the first column. Significant associations between instrumental delivery and smaller transverse diameter were found. The ORs were marginally attenuated after adjustment. Also, a significant association was established between instrumental delivery and a smaller levator hiatus area, both at rest and during contraction, even after adjustment for possible confounders.
      Table 4Logistic regression analysis of association between levator hiatus dimensions and delivery mode
      VariableDelivery mode (normal vaginal delivery vs instrumental delivery)
      Crude OR (95% CI)P valueAdjusted OR (95% CI)P value
      LHapR, cm0.83 (0.51–1.34).450.79 (0.45–1.42).44
      LHrlR, cm0.09 (0.03–0.27)< .0010.10 (0.03–0.33)< .001
      LHareaR, cm20.82 (0.71–0.94)< .010.79 (0.67–0.94)< .01
      LHapC, cm0.99 (0.57–1.73).991.29 (0.67–2.49).44
      LHrlC, cm0.05 (0.01–0.16)< .0010.06 (0.02–0.23)< .001
      LHareaC, cm20.78 (0.65–0.94)< .010.79 (0.64–0.98)< .05
      LHapV, cm0.94 (0.70–1.27).700.90 (0.63–1.28).55
      LHrlV, cm0.45 (0.24–0.84)< .050.43 (0.21–0.86)< .05
      LHareaV, cm20.97 (0.91–1.02).250.96 (0.89–1.03).22
      Levator hiatus dimensions measured at 37 weeks of gestation. Adjustment was made for prepregnancy body mass index (kilograms per square meter), fetal birthweight (grams), duration of total second stage (minutes), gestational length (days), epidural analgesia, labor augmentation, and levator ani muscle cocontraction during the Valsalva maneuver. Each line in the table refers to a unique logistic regression analysis containing no other levator hiatus dimension other than the measure in the first column. adjusted OR, adjusted OR for instrumental intervention during delivery; ap, anteroposterior diameter; C, contraction; CI, confidence interval; crude OR, crude OR estimated by logistic regression analysis for instrumental intervention during delivery; LH, levator hiatus; OR, odds ratio; rl, transverse diameter; R, rest; V, Valsalva maneuver.
      Siafarikas. Levator hiatus dimensions and the process of labor. Am J Obstet Gynecol 2014.
      Levator hiatus dimensions at 22 weeks of gestation showed the same association with delivery outcomes as the findings at 37 weeks of gestation. At 22 weeks of gestation, the levator hiatus area during Valsalva maneuver was significantly higher in women who were later to have normal vaginal deliveries compared with women who later had instrumental deliveries (mean difference, 1.95 cm2; 95% CI, 0.41–3.50; P = .01). Logistic regression analysis revealed an increased adjusted OR of 1.12 (95% CI, 1.01–1.23; P < .05) per additional square centimeter of levator hiatus area during the Valsalva maneuver for normal vaginal delivery. The levator hiatus area during the contraction did not differ between the delivery groups.

      Comment

      In this cohort study of 231 pregnant nulliparous women, larger levator hiatus dimensions at rest and during contraction at 37 weeks of gestation correlated with a shorter duration of the active second stage in women with normal vaginal delivery. The transverse diameter and area of the levator hiatus were larger in late pregnancy in women who went to have normal, as opposed to instrumental, deliveries. Adjustment for confounding factors in logistic regression analysis confirmed that a smaller transverse diameter, and area of levator hiatus, measured at rest and during contraction, was as an independent risk factor for instrumental intervention during delivery.
      The strengths of the study were the relatively large sample size, the fact that few study participants were lost to follow-up, and the minimal variation in gestational length at ultrasound examinations. Render mode was used to measure levator hiatus dimensions, which may be regarded as giving a more representative image of the non-Euclidean shape of the levator ani muscle than measurements in a flat plane, which were used in earlier studies.
      • Kruger J.A.
      • Heap S.W.
      • Murphy B.A.
      • Dietz H.P.
      How best to measure the levator hiatus: evidence for the non-Euclidean nature of the ‘plane of minimal dimensions’.
      To our knowledge, this is the first study to take into account other known factors contributing to delivery outcome. One possible limitation of the study is that no a priori power calculation was performed. Another criticism of our analysis could be that we did not stratify for indication of operative deliveries. We did not do that for 2 reasons. The role of the levator ani muscle in the risk of fetal distress is not yet clear. It has been postulated that poor pelvic compliance might increase intrauterine pressure and predispose fetal distress.
      • Lanzarone V.
      • Dietz H.P.
      Three-dimensional ultrasound imaging of the levator hiatus in late pregnancy and associations with delivery outcomes.
      Furthermore, in our study population, a number of instrumental interventions were carried out for combined indications of fetal distress and failure to progress. Taking out women who underwent an instrumental delivery on the indication fetal distress in a subanalysis, however, did not change results.
      We chose not to exclude women with second-stage cesarean sections because the influence of the levator ani muscle on the delivery process in this group of women is likely. Computer model studies simulating childbirth indicate that the fetal head contacts the levator ani muscle after it has descended 1 cm below the ischial spines.
      • Lien K.C.
      • Mooney B.
      • Delancey J.O.
      • Ashton-Miller J.A.
      Levator ani muscle stretch induced by simulated vaginal birth.
      In our dataset, all 5 women with second-stage cesarean delivery had pushed actively for at least several minutes; 2 of them also underwent attempts at instrumental vaginal delivery (failed forceps), and the fetal head had clearly passed the ischial spines.
      The correlation between smaller levator hiatus dimensions and a longer duration of active second stage of labor in women having a normal vaginal delivery is in line with results from a study by Lanzarone and Dietz,
      • Lanzarone V.
      • Dietz H.P.
      Three-dimensional ultrasound imaging of the levator hiatus in late pregnancy and associations with delivery outcomes.
      in which an inverse correlation between levator hiatus dimensions and the length of the second stage of labor was found.
      Interestingly, in women with a normal vaginal delivery, the size of the anteroposterior diameter and the levator hiatus area were of importance for the duration of active second stage. These measurements are clearly affected by the muscular portion of the levator hiatus, which is bordered by both bony and muscular elements. The correlation of levator hiatus dimensions with the duration of active, but not passive, second stage might have been expected. Computer model studies have demonstrated that it is in the latter stage that the fetal head increasingly interact and stretches the muscle to enable passage.
      • Parente M.P.
      • Natal Jorge R.M.
      • Mascarenhas T.
      • Fernandes A.A.
      • Silva-Filho A.L.
      Computational modeling approach to study the effects of fetal head flexion during vaginal delivery.
      • Lien K.C.
      • Mooney B.
      • Delancey J.O.
      • Ashton-Miller J.A.
      Levator ani muscle stretch induced by simulated vaginal birth.
      • Parente M.P.
      • Natal Jorge R.M.
      • Mascarenhas T.
      • Silva-Filho A.L.
      The influence of pelvic muscle activation during vaginal delivery.
      The absence of a significant correlation between the length of active second stage and levator hiatus dimensions in women with instrumental deliveries might be explained by the iatrogenic shortening of delivery duration. Also, the small sample size in this group could be the reason for the lack of statistically significant results.
      The clearest finding from the logistic regression analysis was the significant association between instrumental intervention during delivery and a smaller transverse diameter of the levator hiatus at 37 weeks of gestation. The transverse diameter was measured as the widest part of the levator hiatus, perpendicular to the anteroposterior diameter.
      • Dietz H.P.
      • Shek C.
      • Clarke B.
      Biometry of the pubovisceral muscle and levator hiatus by three-dimensional pelvic floor ultrasound.
      • Braekken I.H.
      • Majida M.
      • Ellstrom-Engh M.
      • Dietz H.P.
      • Umek W.
      • Bo K.
      Test-retest and intra-observer repeatability of two-, three- and four-dimensional perineal ultrasound of pelvic floor muscle anatomy and function.
      • Majida M.
      • Braekken I.H.
      • Umek W.
      • Bo K.
      • Saltyte Benth J.
      • Ellstrom Engh M.
      Interobserver repeatability of three- and four-dimensional transperineal ultrasound assessment of pelvic floor muscle anatomy and function.
      Both at rest and during contraction, this is normally where the levator ani muscle inserts into the pelvic bone (Figure) and represents the least flexible part of the muscle. A larger, transverse diameter might represent a more lateral insertion of the muscles into the pelvic bone and, from an obstetrical perspective, could indicate a more suitable pelvic outlet. Hence, it is possible that the relationship between levator muscle anatomy and the dimensions of the bony pelvis are of obstetrical importance.
      Results from computational modeling studies show the influence of levator ani muscle mechanics but also of the dimensions of the bony pelvis on the birth process.
      • Parente M.P.
      • Natal Jorge R.M.
      • Mascarenhas T.
      • Silva-Filho A.L.
      The influence of pelvic muscle activation during vaginal delivery.
      • Yan X.
      • Kruger J.
      • Li X.
      • Nash M.
      • Nielsen P.
      Modelling effect of bony pelvis on childbirth mechanics.
      Nevertheless, to date, dimensions of the bony pelvis, as estimated by X-ray pelvimetry or transperineal ultrasound, cannot be used to predict labor outcome.
      • Pattinson R.C.
      Pelvimetry for fetal cephalic presentations at term.
      • Albrich S.B.
      • Shek C.
      • Dietz D.E.
      Can vaginal birth be predicted by antenatal measurement of the subpubic arch angle by 3D transperineal ultrasound?.
      In the present study, a smaller levator hiatus area, at rest and during contraction in late pregnancy, increased the risk of instrumental intervention. However, it is doubtful whether a minimal risk increase, by a factor of 1.2 per additional square centimeter in area, has any clinical relevance.
      Previous published data have not shown a distinct correlation between levator hiatus dimensions during pregnancy and delivery mode.
      • Lanzarone V.
      • Dietz H.P.
      Three-dimensional ultrasound imaging of the levator hiatus in late pregnancy and associations with delivery outcomes.
      • Toozs-Hobson P.
      • Balmforth J.
      • Cardozo L.
      • Khullar V.
      • Athanasiou S.
      The effect of mode of delivery on pelvic floor functional anatomy.
      Lanzarone and Dietz
      • Lanzarone V.
      • Dietz H.P.
      Three-dimensional ultrasound imaging of the levator hiatus in late pregnancy and associations with delivery outcomes.
      found significant larger transverse diameter during contraction in women with normal vaginal, as opposed to instrumental deliveries, but no other levator hiatus dimensions achieved statistical significance between groups. Toozs-Hobson et al
      • Toozs-Hobson P.
      • Balmforth J.
      • Cardozo L.
      • Khullar V.
      • Athanasiou S.
      The effect of mode of delivery on pelvic floor functional anatomy.
      described larger levator hiatus area during the Valsalva maneuver antenatally in women with normal vaginal delivery compared with women with cesarean section using transvaginal ultrasound.
      Surprisingly, in our study the levator hiatus area during the Valsalva maneuver at 37 weeks of gestation was not associated with delivery outcome. One possible explanation could be the fact that the bony related structures of the levator hiatus (transverse diameter) are more associated with delivery mode, than the measurements affected by the muscular portions. The bony part of the levator hiatus contributes much less to the area during the Valsalva maneuver than at rest and during contraction because only the muscular part of the levator hiatus distends. Also, the median levator hiatus area during the Valsalva maneuver is, at 18.0 cm2 in late pregnancy, comparatively small in relation to the required cross-sectional area of approximately 70-90 cm
      • Parente M.P.
      • Natal Jorge R.M.
      • Mascarenhas T.
      • Fernandes A.A.
      • Silva-Filho A.L.
      Computational modeling approach to study the effects of fetal head flexion during vaginal delivery.
      required during the delivery of a fetal head,
      • Lanzarone V.
      • Dietz H.P.
      Three-dimensional ultrasound imaging of the levator hiatus in late pregnancy and associations with delivery outcomes.
      and the area achieved by voluntary Valsalva maneuvers might not represent the potential of the muscle to distend under extreme force. Furthermore, the efficiency of the voluntary Valsalva maneuver was difficult to verify.
      • Shek K.L.
      • Dietz H.P.
      Intrapartum risk factors for levator trauma.
      Even if we tried to minimize bias by a defined procedure in how to perform and analyze Valsalva maneuver and by controlling for cocontraction of the levator ani muscle during the Valsalva maneuver in a regression analysis, the efficiency of the procedure might influence our results.
      In this study we have chosen to focus on measurements collected at 37 weeks of gestation. Significant changes to the levator hiatus during pregnancy have previously been described.
      • Staer-Jensen J.
      • Siafarikas F.
      • Hilde G.
      • Bo K.
      • Engh M.E.
      Ultrasonographic evaluation of pelvic organ support during pregnancy.
      To include these changes to the levator ani muscle in our analysis, we used the measurements of the levator hiatus taken closest in time to delivery, instead of using ultrasound findings taken earlier in pregnancy.
      There is a demand for reliable tools to predict adverse delivery outcomes for both mother and child. Signs of a difficult birth, such as a long second stage and instrumental intervention during vaginal delivery, have been suggested as increasing the risk of injuries to the levator ani muscle,
      • Shek K.L.
      • Dietz H.P.
      Intrapartum risk factors for levator trauma.
      • Valsky D.V.
      • Lipschuetz M.
      • Bord A.
      • et al.
      Fetal head circumference and length of second stage of labor are risk factors for levator ani muscle injury, diagnosed by 3-dimensional transperineal ultrasound in primiparous women.
      one of the major contributors to female pelvic floor dysfunctions.
      • Dietz H.P.
      Clinical consequences of levator trauma.
      • Delancey J.O.
      • Kearney R.
      • Chou Q.
      • Speights S.
      • Binno S.
      The appearance of levator ani muscle abnormalities in magnetic resonance images after vaginal delivery.
      • Allen R.E.
      • Hosker G.L.
      • Smith A.R.
      • Warrell D.W.
      Pelvic floor damage and childbirth: a neurophysiological study.
      In 2006, Dietz et al
      • Dietz H.P.
      • Lanzarone V.
      • Simpson J.M.
      Predicting operative delivery.
      presented predictive models, using clinical data (women's age, family history of caesarean sections, Bishop score) and ultrasound variables (fetal head engagement, cervical length, bladder neck position during Valsalva maneuver) with the aim of identifying women at risk of operative deliveries. Nevertheless, as yet a prospective randomized controlled trial to test the effect of the models in routine clinical use has not been carried out.
      Childbirth is a highly complex process and levator ani muscle anatomy is only 1 factor influencing events. In the present study, differences in levator hiatus dimensions between delivery groups were measured in millimeters, and there were remarkable interindividual variations in the size of the levator hiatus. Clinically it is therefore impossible to draw conclusions from an ultrasound examination of the levator hiatus to distinguish women at high risk of experiencing a prolonged delivery and/or instrumental intervention.
      Our findings imply, however, that it is relevant to include an evaluation of the levator ani muscle when conducting studies related to the birth process, for example when constructing models to predict delivery mode. This study contributes to our understanding of the mechanisms behind the birth and delivery process.

      Acknowledgments

      We thank Kristin Gjestland for the image analysis and midwife Tone Breines Simonsen for recruiting the participants and administering clinical appointments and electronic questionnaires.

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