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The detection of increased nuchal translucency is crucial for the assessment risk of aneuploidies and other fetal anomalies.
Objective
This study aimed to investigate the ability of a transverse view of the fetal head to detect increased fetal nuchal translucency at 11 to 13 weeks of gestation.
Study Design
This was a prospective study enrolling a nonconsecutive series of women who attended our outpatient clinic from January 2020 to April 2021 for combined screening and were examined by operators certified by the Fetal Medicine Foundation. In each patient, nuchal translucency measurements were obtained both from a median sagittal view and from a transverse view. A second sonologist blinded to the results of the first examination obtained another measurement to assess intermethod and interobsever reproducibility.
Results
A total of 1023 women were enrolled. An excellent correlation was found between sagittal and transverse nuchal translucency measurements, with a mean difference of 0.01 mm (95% confidence interval, −0.01 to 0.02). No systematic difference was found between the 2 techniques. The inter-rater reliability (intraclass correlation coefficient, 0.957; 95% confidence interval, 0.892–0.983) and intrarater reliability (intraclass correlation coefficient, 0.976; 95% confidence interval, 0.941–0.990) of axial measurements were almost perfect. Transverse measurements of 3.0 mm identified all cases with sagittal measurements of ≥3.0 with a specificity of 99.7%; transverse measurements of >3.2 mm identified all cases with sagittal measurements of 3.5 mm with a specificity of 99.7%. The time required to obtain transverse nuchal translucency measurements was considerably shorter than for sagittal measurements, particularly when the fetus had an unfavorable position.
Conclusion
When the sonogram is performed by an expert sonologist, the difference in nuchal translucency measurement obtained with a transverse or sagittal plane is minimal. Increased nuchal translucency can be reliably identified by using transverse views, and in some cases, this may technically be advantageous.
In pregnancies undergoing noninvasive prenatal testing, accurate assessment of nuchal translucency (NT) is not necessary for all fetuses, but the identification of those with an excessively large measurement would be important because this should prompt the offer of an invasive procedure.
Key findings
The measurement of NT in the transverse plane identified accurately cases with a sagittal measurement of ≥3.0 mm. The time required to obtain a transverse measurement was shorter than for a sagittal measurement, especially for fetuses with an unfavorable position.
What does this add to what is known?
Increased NT can be reliably identified by using transverse views that are more rapidly obtained than sagittal ones.
Introduction
Providing pregnant patients with an estimation of the risk of fetal aneuploidies by first-trimester screening
An essential part of this examination, the so-called combined test, is the measurement of the nuchal translucency (NT), the hypoechoic area behind the fetal neck, a risk factor for trisomy 21, and a broad range of other chromosomal and nonchromosomal anomalies.
Diagnosis of major heart defects by routine first-trimester ultrasound examination: association with increased nuchal translucency, tricuspid regurgitation and abnormal flow in ductus venosus.
In recent years, noninvasive prenatal testing (NIPT) by cell-free DNA analysis of maternal blood has become increasingly popular and is even offered by national healthcare programs in some countries or areas.
There is a consensus that the detection of an NT >3.5 mm should prompt the offer of an invasive procedure to determine karyotype with chromosomal microarray because, in these cases, there is an increased risk of anomalies that would be otherwise undetectable.
Reexamining the optimal nuchal translucency cutoff for diagnostic testing in the cell-free DNA and microarray era: results from the Victorian Perinatal Record Linkage study.
In brief, accurate measurement is required for a combined test; however, in pregnancies undergoing NIPT, the relevant clinical information is whether the measurement is >3.5 or 3.0 mm, depending on the threshold that is chosen. Thus far, the measurement of the NT has always been obtained with a median sagittal view of the fetal neck, and this requires some effort and expertise. We aimed to evaluate whether transverse scans, which are easier to obtain because less dependent on the fetal position, allow the identification of an excessively enlarged NT.
Materials and Methods
We enrolled a nonconsecutive series of women between 11 0/7 and 13 6/7 weeks of gestation referred to our center from January 2020 to April 2021 for the first-trimester combined screening. Moreover, patients were offered NIPT according to the local policy of the Emilia-Romagna Region.
Patients were offered to participate when one of the sonologists involved in the study was in charge of the examinations. In each case, an operator certified by the Fetal Medicine Foundation obtained the NT measurement using the sagittal image.
In addition, the same operator acquired an axial image of the head using transabdominal or transvaginal ultrasound. All acquisitions were performed using Voluson E8 or E10 systems (General Electric Kretz Ultrasound, Zipf, Austria) with 3 to 7 MHz transducers. Furthermore, the NT was measured in a transverse section offline by a second operator blinded to the result of the combined test and NIPT result, to assess the intermethod agreement. Measurements were subsequently repeated on stored images by the first operator, to assess interoperator reliability. To minimize bias, each operator was blinded to the measurements of the other one.
Sagittal nuchal translucency
The measurement of the translucency was obtained following the recommendation of the Fetal Medicine Foundation positioning the calipers inner to inner.
With the fetus in the neutral position, a median sagittal section of the fetal head and upper thorax was obtained, demonstrating the tip of the nose, with the cheekbone not visible.
Transverse nuchal translucency
A transverse view of the fetal head oriented along the suboccipitobregmatic plane was obtained, similarly to the transcerebellar view described in second-trimester fetuses.
ISUOG Practice Guidelines (updated): sonographic examination of the fetal central nervous system. Part 1: performance of screening examination and indications for targeted neurosonography.
The anatomic landmarks included from anterior to posterior: the frontal horns, optic thalami, and cerebral peduncles. The scanning plane crosses the posterior fossa, which is small and usually poorly demonstrated in the first trimester. The translucency was measured by positioning the calipers inner to outer (on the external contour of the occipital bone and the outer contour of the skin) (Figure 1). For each patient, we compared transverse NT and sagittal NT, and we reported the presence or absence of internal septations (Figure 2). We evaluated the accuracy of transverse assessment to detect a sagittal translucency of >3.5 and 3.0 mm. In a subgroup of 50 cases, we also evaluated the time required for the same operator to obtain a measurement of the sagittal and axial views.
Axial view of fetal head demonstrating the FH, T, and cerebral P. Nuchal translucency is measured from the outer contour of the occipital bone to the external contour of the skin (arrows).
FH, frontal horns; P, peduncles; T, thalami.
Montaguti. Detecting increased nuchal translucency by transverse planes. Am J Obstet Gynecol 2022.
Continuous variables were summarized as mean±standard deviation (SD); discrete and categorical variables were summarized as frequencies and percentages.
A 2-sample t test was used to compare mean sagittal values when axial measurements were undetectable vs detectable. Agreement between transverse and sagittal measurements was assessed using a paired t test and illustrated using the Bland-Altman plot. Inter-rater reliability of measurements obtained in the transverse plane was assessed with intraclass correlation coefficient (ICC) estimates and 95% confidence intervals (CIs) based on a single-rating, absolute-agreement, 2-way random-effects model. Intrarater reliability of measurements made by operator 1 was assessed with ICC estimates and 95% CIs based on a single-rating, absolute-agreement, 2-way mixed-effects model. This analysis was performed on a random subsample of 20 subjects.
The sensitivity and specificity of transversal measurements relative to sagittal measurements (gold standard) were evaluated using both 3.0 and 3.5 mm as the thresholds to define a positive result. Of note, 95% CIs for both accuracy indices were calculated with the Wilson score method.
Agreement between the 2 ultrasound methods in the identification of internal septations was assessed with Cohen’s κ. All data were analyzed using the Stata 15 software (Release 15; StataCorp LP, College Station, TX).
Ethics
The study protocol conforms to the ethical guidelines of the “World Medical Association (WMA) Declaration of Helsinki-Ethical Principles for Medical Research Involving Human Subjects” adopted by the 18th WMA General Assembly, Helsinki, Finland, June 1964 and amended by the 59th WMA General Assembly, Seoul, Republic of Korea, October 2008. The study protocol was approved by the local ethics committee of Sant’Orsola-Malpighi Hospital (203/2020/Oss/AOUBo), and a consent form signed at recruitment was obtained from each eligible patient.
Results
We enrolled a sample of convenience of 1023 women, whose characteristics are reported in Table 1. In 45 patients (4.4%) a high risk of trisomy 21, 13, or 18 was calculated from either the combined test or NIPT. Of those patients, 33 (73%) decided to undergo chorionic villous sampling. In 27 cases (82%), the karyotype was normal, whereas 6 cases (18%) had aneuploidies (3 cases of trisomy 21, 1 case of trisomy 18, and 1 case of trisomy 9 and 16, respectively).
bHCG, free fraction of human chorionic gonadotropin; BMI, body mass index; CRL, crown-rump length; MoM, multiple of the median; NT, nuchal translucency; PAPP-A, pregnancy-associated plasma protein-A; SD, standard deviation.
Montaguti. Detecting increased nuchal translucency by transverse planes. Am J Obstet Gynecol 2022.
In 8 fetuses, intranuchal septations were seen in both transverse and sagittal planes (Figure 2); the mean sagittal NT in these cases was 6.3±1.8 mm (range, 4.4–9.9), whereas the mean transverse NT was 6.7±2.0 mm (range, 3.9–10.3). The 2 ultrasound methods fully agreed on identifying internal septations (Cohen’s κ=1).
In 310 cases (30%) in the transverse plane, only the echogenic line of the skin was visible posterior to the occipital bone, without any measurable NT (Figure 3). In these cases, the mean sagittal thickness was 1.26±0.25 mm (range, 0.50–2.10). In those cases in which a transverse NT measurement could be obtained, the mean difference from the sagittal one was 0.01 mm (95% CI, −0.01 to 0.02). In these cases, there was no systematic difference between the 2 techniques (paired t test, 1.09; P=.277). As shown in Figure 4, there was no evidence of proportional bias, that is, the 2 methods agreed equally through the range of measurements. The 95% limits of agreement illustrated in the aforementioned figure, which are defined as the mean difference ±1.96 times the SD of the differences and indicate how far apart measurements are likely to be for most individuals, were −0.39 mm (95% CI, −0.42 to −0.36) and 0.41 mm (95% CI, 0.38–0.44). The inter-rater reliability (ICC, 0.957; 95% CI, 0.892–0.983) and intrarater reliability (ICC, 0.976; 95% IC, 0.941–0.990) of NT thickness obtained in the transverse plane were almost perfect.
Short-dashed lines indicate the 95% CI for the mean difference. Dotted lines indicate the 95% CIs of the limits of agreement. Notes: 294 patients with undetectable NT thickness in the axial plane were excluded (n=729).
CI, confidence interval; NT, nuchal translucency; SD, standard deviation.
Montaguti. Detecting increased nuchal translucency by transverse planes. Am J Obstet Gynecol 2022.
The accuracy of different thresholds of transverse NT to predict an enlarged sagittal sonolucency is reported in Table 2. In particular, an axial measure of ≥3.2 mm predicted a sagittal NT of ≥3.5 mm with a sensitivity of 100.0% (95% CI, 78.5–100.0), a specificity of 99.7% (95% CI, 99.0–99.9), and a positive predictive value of 87.5% (95% CI, 64.0–96.5), whereas a transverse measure of ≥3.0 mm predicted a sagittal NT of ≥3.0 mm with a sensitivity of 100% (95% CI, 80.6–100.0), a specificity of 99.7% (95% CI, 99.0–99.9), and a positive predictive value of 88.9% (95% CI, 67.2–96.9).
Table 2Sensitivity and specificity of fetal nuchal translucency thickness obtained in the axial plane using incremental thresholds to define a positive result
Axial NT (mm)
Sagittal NT>3.0 mm
Sagittal NT>3.5 mm
Sensitivity (95% CI)
Specificity (95% CI)
Sensitivity (95% CI)
Specificity (95% CI)
2.9
100.0 (80.6–100.0)
99.3 (98.4–99.7)
100.0 (78.5–100.0)
99.0 (98.0–99.5)
3.0
100.0 (80.6–100.0)
99.7 (99.0–99.9)
100.0 (78.5–100.0)
99.4 (98.6–99.8)
3.1
100.0 (80.6–100.0)
100.0 (99.5–100.0)
100.0 (78.5–100.0)
99.7 (99.0–99.9)
3.2
100.0 (80.6–100.0)
100.0 (99.5–100.0)
100.0 (78.5–100.0)
99.7 (99.0–99.9)
3.3
93.8 (71.7–98.9)
100.0 (99.5–100.0)
92.9 (68.5–98.7)
99.7 (99.0–99.9)
3.4
93.8 (71.7–98.9)
100.0 (99.5–100.0)
92.9 (68.5–98.7)
99.7 (99.0–99.9)
3.5
81.3 (57.0–93.4)
100.0 (99.5–100.0)
85.7 (60.1–96.0)
99.9 (99.2–100.0)
Sagittal thickness is used as the gold standard.
CI, confidence interval; NT, nuchal translucency.
Montaguti. Detecting increased nuchal translucency by transverse planes. Am J Obstet Gynecol 2022.
The time required to obtain a transverse NT measure was significantly shorter than for a sagittal one (68±40 seconds vs 101±81 seconds; P=.001), in particular when the fetus had the back upward (89±47 seconds vs 182±70 seconds; P<.001). When the fetus was lying on its side or back, the time for the 2 acquisitions was comparable (54±27 seconds vs 48±13 seconds; P=.098).
Discussion
Principal findings of the study
Our study suggested that measurement of the NT in transverse planes is reproducible and closely correlated with measurement in sagittal planes. In particular, the former accurately predicts excessive values of the latter. In 30% of fetuses, the NT could not be visualized in the transverse view, but in all these cases, the sagittal assessment was always within normal limits. An axial translucency of <3.0 and 3.2 mm effectively ruled out a sagittal translucency of ≥3 and 3.5 mm, respectively. The presence of internal septations was consistently associated with a sagittal NT of ≥3.5 mm.
Results in the context of what is known
The main strength of our paper was that this study compared transverse NT and sagittal NT in a large prospective series during the first trimester of pregnancy.
Clinical and research implications
Sagittal NT has become the standard of care for the quantitative assessment of the NT,
and we are not proposing that this should be replaced by transverse planes. However, we have found that the latter are feasible and reproducible and may be considered in those examinations in which the indication is not the calculation of the risk of aneuploidy but rather the detection of an excessively large translucency, as is the case with patients undergoing NIPT.
Depending on the chosen threshold, the need to obtain a sagittal view could be limited to those cases in which an excessive measurement of the transverse NT is obtained.
Strengths and limitations
The rationale of our study is that measurement of transverse NT does not require precise alignment of the sound beam with the midline of the fetus and should be therefore easier to obtain particularly by a less experienced operator. However, we cannot prove this with certainty, as the scans were always obtained by sonologists certified by the Fetal Medicine Foundation and with a long-standing experience. We admit that this was a weakness of our study. However, we believe that we have at least demonstrated that transverse NT can be obtained more rapidly. In the sagittal view, there is a clear demarcation of the NT, and this allows to position the calipers inner to inner and to obtain a precise measurement of the sonolucent space. In the transverse plane, the skin line is often blurred because of the limited lateral resolution of the ultrasound beam. Therefore, we decided to include the skin in the measurement. Although it could be objected that this represents an indirect quantification of sonolucency, we have found that the measurements were reproducible and very well correlated with those obtained in the sagittal plane. In about one-third of cases in the transverse plane, no anechoic space was seen posteriorly to the occipital bones. In all these fetuses, the standard sagittal measurement was well within normal limits.
Conclusions
Here, an axial measurement of <3.0 and <3.2 mm effectively ruled out a sagittal translucency ≥3.0 and 3.5 mm, respectively, with very high specificity. This would allow to greatly reduce the number of cases in which a sagittal view of the fetus needs to be obtained.
Diagnosis of major heart defects by routine first-trimester ultrasound examination: association with increased nuchal translucency, tricuspid regurgitation and abnormal flow in ductus venosus.
Reexamining the optimal nuchal translucency cutoff for diagnostic testing in the cell-free DNA and microarray era: results from the Victorian Perinatal Record Linkage study.
ISUOG Practice Guidelines (updated): sonographic examination of the fetal central nervous system. Part 1: performance of screening examination and indications for targeted neurosonography.
Cite this article as: Montaguti E, Rizzo R, Diglio J, et al. Increased nuchal translucency can be ascertained using transverse planes. Am J Obstet Gynecol 2022;227:750.e1-6.
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