Fetal Genitalia: First Trimester

Sonographic determination of fetal sex can be divided into early and late gestation. Fetal sex determination in the first trimester is based on both the direction of the genital tubercle relative to the lumbosacral skin surface (Genital tubercle angle – GTA) as well as the distance from the anus to the genitalia, the anogenital distance (AGD), while in the second and third trimesters it is based on direct visualization of internal or external genitalia. 

Both the AGD and GTA demonstrate significant sexual dimorphism.

Prenatal sex assignment at the time of the first trimester ultrasound, although less straightforward than second or third trimester sonography, has a high accuracy rate that improves with advancing gestational age.

Sex differentiation is not fully completed until about the 11^th week of gestation, and therefore, sex determination on ultrasound is relatively inaccurate prior to the 12^th week of gestation. After 12 wks a penile shaft may be differentiated from a clitoris (Figures 1a,b) as the linear parallel folds of the labia start becoming visible as separate structures (Figures 1c, d).  Emerson et. al. (1989) were the first to identify the ‘sagittal sign’ as a means of determining genitalia at first trimester ultrasound (1). On a midline sagittal view of the caudal end of the fetal torso, the contour of the rump is followed from dorsal to ventral until a focal bulge with an angular notch is found. This study found that a focal bulge creating a cranial acute angle indicated male genitalia, while a bulge creating a caudal acute angle indicated female genitalia. Orientation of the genitalia is used for sex differentiation as prior to 14 weeks of gestation there may be no appreciable difference in size between the penis and clitoris.

The accuracy using this sign increases between 10 and 20 weeks of gestation and has been shown to be most accurate between 14 and 20 weeks of gestation with a correct sex assignment of 75% between 12 and 14 weeks of gestation and nearly 100% after 14 + 0 weeks of gestation (1).

This technique was expanded (2) by measuring the angle of the genital tubercle (GTA) relative to the lumbosacral skin surface. The fetal sex is assigned as male if the angle is greater than 30° and female if the phallus is parallel or convergent (less than 30°) to the horizontal line. The accuracy of this technique increases with fetal crown–rump length and gestational age and has a success rate between 70% at 11 weeks and 100% at 13 weeks.   

These findings were validated in a more recent study that found the female genital tubercle points inferiorly (caudally) with a mean angle of 21.57 degrees, and 35.90 degrees in males, with a sensitivity of 92%, specificity of 84.7%, and an AUC of 0.932 (95% CI: 0.904-0.961) using 28.32 degrees as a cutoff point (3) 

Determination of the angle of the genital tubercle in female fetuses may however be challenging when either the angle is converging, or when the tubercle is parallel to the lumbosacral surface.

Ultrasound: Although the genitalia can be seen in either an axial plane (Figure 1) or midsagittal plane (Figures 2 and 3) of the pelvis, sonographic assessment of the genital region is usually performed in a mid sagittal plane (the same plane that is used for the CRL measurement).

 Technique:

• The fetus is horizontal (parallel) to the probe in a supine position (no extension of the limbs or spine).
• A mid sagittal image is acquired and the angle of the genital tubercle to a horizontal line through the lumbosacral skin surface is measured (Figures 2, 3).
• By measuring the angle of the caudal or cranial notch in the mid-sagittal plane, the fetal sex is assigned male if the angle is greater than 30 degrees (cranially directed phallus) and female if the phallus is parallel or convergent (less than 10 degrees) to the horizontal line (caudally directed clitoris). At an intermediate angle of 10-30 degrees fetal sex is not assigned. In the study conducted by Youssef, et al. the best cut-off for male sex determination was found to be between 27° and 29° (5). This angle is called the genital tubercle angle (GTA).
• Using this method, assignment of fetal sex has a high accuracy rate at 12-14 weeks (3).
• The accuracy of female genitalia assignment increases with increasing CRL (crown rump length).
• The accuracy of male genitalia assignment does not appear to change significantly with increasing CRL.
• The advantages of utilizing the genital tubercle angle measurement includes both the familiarity of the imager in obtaining the mid-sagittal plane, which is the same plane as for the nuchal translucency measurement, and the relatively small amount of extra time required to perform this measurement.

Efrat et al. (2006) (4) found in their study that the overall sensitivity was higher in identifying males (99.6%) than females (97.4%). Additionally, after 12 weeks' gestation, the accuracy of correctly identifying a male fetus approaches 100%.  They recommended waiting until the CRL was at least 62.5 mm prior to proceeding with invasive testing as accuracy improved with increasing CRL (4).

Youssef et al (2011) ascertained the feasibility of fetal sex determination utilizing 3D ultrasound to measure the genital tubercle angle in 85 cases, with a 100% accuracy (5).

However, Bogers et al in 2018 (6) only achieved a 56% accuracy for fetal sex determination with 3D ultrasound in the first trimester and concluded that even when using advanced 3D techniques accurate results were not possible.

Kearin et.al. (2014) demonstrated a 100% accuracy in predictions made after 14 weeks gestation (7). The overall success rate in the first trimester group (11-14 weeks) was 75%. When excluding those scans where a prediction could not be made, success rates increased to 91%. Results were less accurate for fetuses younger than 12 weeks, with an overall success rate of 54%. Male fetuses under 13 weeks were more likely to have sex incorrectly reported or unable to be assigned. After 13 weeks, success rates for correctly predicting males exceeded that of female fetuses.

A systematic review and meta-analysis (8) of 9 studies reported the accuracy rate of first-trimester fetal sex identification, with a mean accuracy of 85% and a median of 87%. A pooled sensitivity and specificity analysis shows that the sensitivity increased from 69% at 11 weeks to 89% at 12 weeks to 96% at 13 weeks. They found that male sex is more easily detected in the second and third trimesters, while female sex is more easily detected in the first trimester. The sensitivity of fetal sex detection in the first trimester increases with gestational age.

AGD: This distance is considered another method of sonographically determining fetal sex in the first trimester.

The AGD measures the distance from the caudal extremity of the fetus - anus to the genital tubercle (Figures 4 and 5), with males having a longer AGD than females as it is testosterone dependent.

In 2016, Arfi et al (9). reported sensitivities for determination of male sex at 87% and female sex at 89% by measuring the AGD in 310 fetuses at 11-14 weeks' gestation using a cut-off of 4.8mm (male , ≥4.8mm) and female <4.8mm). The inter-observer variability was excellent.

Najdi et al. (10) reported in 2019 their experience with using the AGD to determine fetal sex in 316 cases. They found the best cut-off for 11 wk to 11 wk 6 days of pregnancy was 4.5 mm, for 12 wk to 12 + 6 wks and 4.9 mm for 13 wk to 13 +6 wks Their accuracy increased from 70.4% at 11 weeks' gestation to 96.0% at 12 weeks' gestation and 100% at 13 weeks gestation. However, the threshold AGD for sex determination in this study was gestational age dependent, increasing the complexity of this technique.

In a recent study, the AGD had a mean length of 7.16 mm in male fetuses and 4.42 mm in female fetuses, with a sensitivity of 88.8%, specificity of 94.4%, and an area under the ROC curve (AUC) of 0.931 (95% CI: 0.899-0.962) using 5.74 mm as a cutoff point (6). At birth, AGD serves as a non-invasive indicator of androgen levels and can predict abnormal reproductive system development (7,9).

Despite the high predictive accuracy of AGD, normal reference ranges vary between studies, ranging from 3.6 mm to 5.92 mm in females and from 5.10 to 7.16 mm in males (9-11). As a result, the suggested optimal cutoff values varied from 4.5 mm to 6.0 mm (3). The variations in AGD’s reference ranges across different populations highlight the potential influence of racial differences, genetic and environmental factors on AGD measurements and suggest the need for population-specific reference ranges. Therefore, optimal cutoff values for sex determination should be race-specific to ensure accuracy.

A study by Alfuraih et.al. (11) demonstrated findings that align closely with former studies, indicating a high sensitivity and specificity of AGD for fetal sex determination and suggested that AGD measurements could accurately predict fetal sex with high accuracy, and is a reliable marker for early sex determination during the first trimester. Currently the AGD and GTA are considered reliable markers for early fetal sex determination, with AGD showing higher reproducibility (6).

Sipahi et al. (12) demonstrated that AGD measurement could predict fetal sex with 76.7% sensitivity and 95.6% specificity. Measurements of AGD ≥4.8 mm were identified for males, and AGD <4.8 mm for females.

The AGD is reported to have a higher accuracy than GTA in distinguishing male from female fetuses using a cut-off value of 4.8 mm.

Some workers also suggest that the AGD is more accurate than the GTA as erroneous female sex assignment reports have been documented using the GTA in cases of genital hypospadias where the tubercle could orient downward in male fetuses (ventral curvature of the penis) (13).  The same problem may occur using the GTA technique by misclassifying female fetuses as male in cases of transient hypertrophy of the labia minora (14, 15).

Incorporating AGD and GTA as non invasive, highly reproducible tests into routine first-trimester ultrasound screenings, can improve the accuracy of early fetal sex determination. However, due to the variability in AGT reference ranges across populations, race specific reference ranges need to be created before this can be considered a highly reliable marker.

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11. Alfuraih, A.M. Alotaiby, S.A. Alsaadi, M.J. et.al. Predictive Value and Reference Ranges of Anogenital Distance for Determining Fetal Gender in the First Trimester. Saudi Med. J. 2021, 42, 1057.
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15. Mallmann MR, Gembruch U. Clinical spectrum of female genital malformations in prenatal diagnosis. Arch Gynecol Obstet. 2022;306(6):1847-1862.

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