Fetal Sex Determination by Ultrasound
Fetal sex can be reliably identified by ultrasound starting at 12 weeks of gestation, with accuracy improving significantly from 11 to 13 weeks, reaching 98-99% accuracy by 13 weeks and beyond.
Optimal Timing for Sex Determination
The accuracy of fetal sex determination by ultrasound is highly gestational age-dependent:
First Trimester (11-14 weeks)
- 11 weeks: Accuracy ranges from 71.9% to 88.3% 1, 2
- 12 weeks: Accuracy improves to 89-94.7% 3, 1, 2
- 13 weeks: Accuracy reaches 96-98.6% 3, 1, 4, 2
The sensitivity for sex detection increases progressively: 69% at 11 weeks → 89% at 12 weeks → 96% at 13 weeks 3.
Second and Third Trimesters (18+ weeks)
- Accuracy reaches 92-99% 3
- The standard anatomic survey is typically performed at 18-20 weeks 5
- Sex determination at this stage is highly reliable as part of routine anatomic assessment
Technical Approach
The technique involves obtaining a mid-sagittal view of the genital region and measuring the angle of the genital tubercle to a horizontal line through the lumbosacral skin surface 1, 4, 6:
- Male assignment: Angle >30 degrees
- Female assignment: Angle <10 degrees (parallel or convergent)
- Indeterminate: Angle 10-30 degrees (sex should not be assigned)
Factors Affecting Accuracy
Several factors influence the feasibility and accuracy of sex determination:
- Crown-rump length (CRL): Best accuracy achieved with CRL >55.7 mm or >62.6 mm 4, 6
- Maternal body mass index: Accuracy decreases with BMI >23.8 6
- Gestational age: Success rate for sex assignment increases from 85% at 11 weeks to 96-97% at 13 weeks 4
Sex-Specific Detection Patterns
Interestingly, detection accuracy varies by fetal sex and gestational age 3:
- First trimester: Female sex is more easily detected
- Second/third trimester: Male sex is more easily detected
- Overall male detection accuracy is slightly higher (92.5% vs 91.2%) but not statistically significant 2
Clinical Recommendations
For clinical decision-making requiring accurate sex determination (such as sex-linked genetic disorders requiring chorionic villus sampling), delay assessment until at least 12+4 weeks or CRL >62.6 mm 4. At 13 weeks, the 98-99% accuracy approaches that of invasive testing 2.
For routine anatomic surveys in low-risk pregnancies, sex determination is typically performed at the standard 18-20 week anatomy scan, where accuracy is consistently >99% 5.
Common Pitfalls
- Avoid making clinical decisions based on sex determination before 12 weeks, particularly for female assignments at CRL <62.6 mm, where the 91.5% accuracy rate leaves meaningful room for error 4
- In obese patients, consider that visualization may be suboptimal; the anatomic survey (including sex determination) should be performed at 20-22 weeks 5
- Do not assign sex when the genital tubercle angle is intermediate (10-30 degrees); wait for a later scan 1, 4