Ultrasound determination of fetal sex can benefit decision-making regarding invasive prenatal testing in pregnancies at risk of sex-linked genetic abnormalities. The aim of this study was to assess the accuracy of fetal sex determination by ultrasound at 12–14 weeks of gestation in a large cohort.
Fetal gender assessment by transabdominal ultrasound was performed in 656 singleton pregnancies at 12–14 weeks of gestation. The genital region was examined in the mid-sagittal plane. The angle of the genital tubercle to a horizontal line through the lumbosacral skin surface was measured. The fetus was assigned male gender if the angle was > 30°, and female gender if the genital tubercle was parallel or convergent (<10°) to the horizontal line. At an intermediate angle of 10–30 degrees the gender was not determined. Crown–rump length (CRL) was measured in all cases.
Gender assignment was possible in 613 of the 656 (93%) fetuses. Gender identification according to CRL was feasible in 85%, 96% and 97% of the fetuses at gestational ages of 12 to 12 + 3, 12 + 4 to 12 + 6 and 13 to 13 + 6 weeks, respectively. Phenotypic sex was confirmed in 555 newborns. The accuracy of male gender assignment in this group was 99–100% at all ages, and that of female gender assignment was 91.5% at 12 to 12 + 3 weeks, 99% at 12 + 4 to 12 + 6 weeks and 100% at 13 to 13 + 6 weeks.
Early gender identification by ultrasound may help prevent unnecessary invasive procedures such as chorionic villus sampling in patients at risk of X-linked diseases. However, there are only a few published studies regarding the accuracy of gender assignment in early pregnancy, most on a relatively small series of patients1–9. The aim of this study was to evaluate the accuracy of gender assignment by transabdominal ultrasound in a large number of fetuses at 12–14 weeks' gestation, and to identify the crown–rump length (CRL) at which the accuracy level is high enough to make a definitive decision regarding invasive procedures in a fetus identified as female.
Fetal gender assessment by transabdominal ultrasound was performed by a single operator (Z.E.) in 656 consecutive singleton pregnancies at 12–14 weeks of gestation (CRL, 55.4–83.9 mm) between February 1998 and December 2002. An HDI 3000 or 5000 ultrasound machine (Advanced Technology Laboratories, Bothell, WA, USA), equipped with a 4–7-MHz convex transducer was used for all scans. The fetal genital region was examined in the mid-sagittal plane with the fetus lying in a natural position (neither hyperflexed nor hyperextended). The angle of the genital tubercle to a horizontal line through the lumbosacral skin surface was measured on a paper image using a protractor. The fetus was assigned male gender if the angle was > 30° (Figure 1), and female gender if the genital tubercle was parallel or convergent (<10°) to the horizontal line (Figure 2). Cases in which the angle was intermediate (10–30°) were classified as undetermined. CRL was evaluated in all cases. The findings on ultrasound were compared with phenotypic sex after birth.
Gender identification according to CRL is presented in Table 1. Gender assignment was possible in 613 of the 656 (93%) fetuses. In 43 cases (7%), gender was undetermined because of maternal habitus/fetal lie (53%) or an intermediate angle of the genital tubercle to the horizontal plane (47%). In 58 of the 613 cases (9.5%) in which gender was sonographically assigned, information on phenotypic sex at birth was unavailable (early miscarriage or termination of pregnancy, n = 9; lost to follow-up, n = 49). Of the remaining 555 fetuses, sex was correctly determined by ultrasound in 99.6% of the males and 97.4% of the females (Table 2). The accuracy of female assignment increased with increasing CRL: it was 91.5% at a CRL of 55.4 to 62.5 mm (12 to 12 + 3 weeks); 99% at a CRL of 62.6 to 67.9 mm (12 + 4 to 12 + 6 weeks), and 100% at a CRL of 68.0 to 83.9 mm (13 to 13 + 6 weeks). This difference was statistically significant (P = 0.045 for CRL 55.4–62.5 mm vs. CRL 62.6–67.9 mm, and P < 0.003 for CRL 55.4–62.5 mm vs. CRL 68.0–83.9 mm). The accuracy of male assignment did not change significantly with an increase in CRL.
Table 1. Gender identification according to crown–rump length (CRL)
Gestational age (weeks)
Gender identified by ultrasound (n (%))
Known gender at birth (n)
Lost to follow-up (n)
12 to 12 + 3
12 + 4 to 12 + 6
13 to 13 + 6
Table 2. Accuracy of sonographic determination of fetal gender
According to our results, gender assignment by ultrasound has a high accuracy rate at 12–14 weeks (99.6% for males and 97.4% for females) and appears to be an effective auxiliary tool in decision-making with regard to invasive procedures. The gender could not be identified in only 7% of cases.
A previous study found that fetal gender assignment was inaccurate at 11 weeks' gestation, with 56% of males being wrongly assigned4. Therefore, we sought to determine the gestational age at which it would be reasonably safe to make decisions regarding the need for invasive testing procedures according to fetal gender. We used a modified version of the method described previously4, wherein the finding of an intermediate angle of 10–30° from the genital tubercle to the horizontal plane precluded assignment of gender.
Mazza et al. in two studies8, 9 aimed to establish the accuracy of fetal gender assignment by sonography related to biparietal diameter (BPD). In the larger study of 2593 patients9, at a BPD of 22 mm or greater, the accuracy rate was 99–100%. However, comparing our results with theirs cannot be accurate since, as stated by Mazza et al., different size charts used to convert BPD to gestational age show discordant results. For example, a BPD of 22 mm corresponds to a mean of 12 + 1 weeks of gestation according to Lasser et al.10 but to 13 + 0 weeks according to Altman and Chitty11. Another study on a much smaller series of fetuses (n = 32) reported that absolute accuracy in gender prediction was achieved at 69 days from fertilization, corresponding to a gestational age of 11 + 6 weeks based on the last menstrual period2. Whitlow et al.1 found that the ability of the operator to assign fetal gender improved significantly with increasing gestational age, from 59% at 11 weeks to 87%, 92% and 98% at 12, 13 and 14 weeks, respectively. Interestingly, in contrast to our study, the accuracy of identifying correctly fetal gender did not change significantly with gestational age. Our present study, in a large cohort, suggests that 12 weeks' gestation (CRL > 55.4 mm) is a safe cut-off for decision-making in fetuses identified sonographically as males. However, in fetuses identified as females, despite the relatively high 91.5% accuracy rate at 12 to 12 + 3 weeks (CRL, 55.4–62.5 mm), the decision to avoid invasive testing should be postponed until a higher CRL is achieved. As a precaution, the gender of the fetus should be reconfirmed later in the course of the pregnancy, preferably at up to 20 weeks' gestation. One should also bear in mind the rare possibility of discordance between phenotypic and genotypic gender, as in testicular feminization or severe hypospadias.