Fetal Sex Determination on Ultrasound
Fetal sex can be reliably determined by ultrasound starting at 13 weeks' gestation, with accuracy rates of 99-100% after this point, though determination may be attempted as early as 11-12 weeks with progressively lower accuracy. 1, 2
Optimal Timing for Clinical Decision-Making
After 13 weeks' gestation, fetal sex determination is accurate in 99-100% of cases when external genitalia are normally formed, making this the earliest clinically reliable timepoint. 1
Between 13+2 to 14+1 weeks, accuracy reaches 98.6%, representing the most reliable window in the first trimester for sex-dependent clinical decisions such as chorionic villus sampling for sex-linked genetic conditions. 3
Attempts before 12 weeks' gestation are relatively inaccurate and should be avoided for clinical decision-making, though feasibility improves with advancing gestational age even within the 11-14 week window. 1
Accuracy by Gestational Age
Early First Trimester (11-12 weeks)
At 11+2 to 12+1 weeks, overall accuracy is only 88.3%, with particular difficulty in female sex assignment (91.5% accuracy) compared to male assignment (99-100% accuracy). 2, 3
Sex determination is feasible in only 85% of fetuses at 12 to 12+3 weeks, limiting its clinical utility at this early stage. 2
A crown-rump length (CRL) threshold of >55.7 mm or gestational age >12+2 weeks significantly improves accuracy, suggesting that clinical decisions should be delayed until these parameters are met. 4
Mid-to-Late First Trimester (12-14 weeks)
Between 12+4 to 12+6 weeks, feasibility increases to 96% with 99% accuracy for both male and female assignment, making this a reasonable window for sex determination when clinically indicated. 2
At 13 to 13+6 weeks, feasibility reaches 97% with 100% accuracy for female assignment and 99-100% for male assignment, establishing this as the optimal first-trimester window. 2
Overall accuracy across 12+2 to 13+1 weeks is 94.7%, representing a practical compromise between early determination and reliability. 3
Technical Methodology
Sonographic Technique
The "sagittal sign" is the primary method for first-trimester sex determination: the genital tubercle is visualized in the mid-sagittal plane, with its angle measured relative to a horizontal line through the lumbosacral skin surface. 2, 3
Male sex is assigned when the genital tubercle angle exceeds 30 degrees (cranially directed phallus), while female sex is assigned when the angle is <10 degrees (parallel or convergent/caudally directed). 2, 3
Angles between 10-30 degrees are indeterminate, and sex should not be assigned in these cases to avoid errors. 2, 3
Second-Trimester Confirmation
After 13 weeks in the late first and early second trimester, direct visualization of external genitalia becomes the standard method, supplementing or replacing the sagittal sign approach. 1
Additional landmarks for male identification include the "dome sign" (fetal scrotum), longitudinal raphe at the base of the penis, and descended testis; for female identification, look for 2-4 parallel lines representing labial folds. 5
Factors Affecting Accuracy
Maternal Factors
- Maternal body mass index (BMI) significantly impacts accuracy, with a BMI <23.8 identified as the optimal cutoff for reliable sex determination; higher BMI reduces visualization quality. 4
Fetal Factors
Fetal CRL is the strongest predictor of accuracy, with CRL >62.6 mm (approximately 13 weeks) providing the most reliable determinations. 2, 4
Malformed external genitalia will result in inaccurate sex determination regardless of timing, as the method relies on normal anatomic development. 1
Operator Experience
- Accuracy improves substantially with operator experience: in one series, male gender identification accuracy increased from 91.7% to 99.7% and female identification from 93.3% to 100% with accumulated experience. 5
Clinical Pitfalls and Recommendations
Common Errors to Avoid
Do not rely on phallus length for sex determination in the first trimester, as this is not diagnostic at early gestational ages and represents the main pitfall for inexperienced sonographers. 5
Avoid making clinical decisions based on female sex assignment at CRL <62.6 mm (before approximately 13 weeks), as the 91.5% accuracy rate at this stage leaves an unacceptable 8.5% error rate for critical decisions. 2
Do not attempt sex determination before 12 weeks' gestation for clinical purposes, as the combination of low feasibility and reduced accuracy makes it unreliable. 1
Optimal Clinical Strategy
For pregnancies at risk of sex-linked genetic abnormalities requiring invasive testing, delay the procedure until 13 weeks or later when male sex can be confirmed with 99-100% accuracy. 2, 3
If a fetus is identified as female before 13 weeks, postpone clinical decisions until a higher CRL is achieved or consider repeat scanning at 13+ weeks for confirmation. 2
Between 13-14 weeks, sex determination has sufficient accuracy (98.6%) to guide clinical decisions such as whether to proceed with chorionic villus sampling in at-risk pregnancies. 3