Fetal Sex Determination on Ultrasound
Fetal sex can be accurately determined on ultrasound after 12 weeks' gestation by visualizing the external genitalia in a mid-sagittal plane, with accuracy exceeding 99% after 13 weeks when using the genital tubercle angle and direct visualization of genital structures. 1, 2
Optimal Timing for Sex Determination
- Do not attempt fetal sex determination before 12 weeks' gestation, as accuracy is relatively poor at this early stage 1
- Between 12-13 weeks, accuracy ranges from 94.7% to 98.7% 2, 3
- After 13 weeks' gestation, accuracy reaches 99-100% in fetuses without malformed external genitalia 1, 4, 2
- Success rates for obtaining adequate images are approximately 97% for both sexes after 12 weeks 4
Technical Approach: The Sagittal Sign Method
Positioning and Image Acquisition
- Obtain a mid-sagittal view of the fetal genital region in the lower pelvic area 1, 2
- Transabdominal ultrasound is the standard approach during the second trimester anatomy scan at 18-22 weeks 5
- Transvaginal ultrasound can be used as an adjunct for early first trimester assessment (13-16 weeks) when transabdominal views are suboptimal 6
Measurement Technique (11-14 Weeks)
- Measure the angle of the genital tubercle relative to a horizontal line through the lumbosacral skin surface 2, 3
- Male assignment: Angle >30° (cranially/upward directed genital tubercle) 2, 3
- Female assignment: Angle <10° (parallel or convergent/caudally directed genital tubercle) 2, 3
- Indeterminate: Angles between 10-30° should not be assigned a sex 3
Direct Visualization Criteria (After 13 Weeks)
Male Fetus Identification
- "Dome" sign: Visualization of the fetal scrotum 6
- Cranially directed phallus (penis pointing upward) 1, 6
- Longitudinal midline raphe at the base of the penis 1, 6
- Descended testis may be visible 1
- Direction and origin of the fetal micturition jet in males 1
Female Fetus Identification
- Two or four parallel lines representing the labial folds 1, 6
- Caudally directed phallus (clitoris pointing downward) 1, 6
- Visualization of the fetal uterus may be possible 1
Clinical Context: When Sex Determination Matters
Multiple Gestations
- Detection of discordant external genitalia (one male and one female) definitively indicates dizygotic, dichorionic twins 5
- This is particularly useful when chorionicity determination is difficult in the second trimester due to membrane thinning or apparent placental fusion 5
- Chorionicity assessment is most accurate in the first trimester, but discordant sex provides definitive confirmation of dichorionicity when identified 5
Routine Anatomy Scan
- Fetal anatomy scan is recommended at 18-22 weeks' gestation to evaluate for congenital malformations 5
- Sex determination is part of the comprehensive anatomic survey during this examination 5
Common Pitfalls and How to Avoid Them
Early Gestation Errors (11-14 Weeks)
- The length of the fetal phallus is NOT diagnostic at this early stage and is the main source of error for inexperienced sonographers 6
- Male fetuses are frequently misidentified as female at 11 weeks (56% error rate), but this drops to 3% at 12 weeks and 0% at 13 weeks 2
- Female fetuses have a lower false-positive rate (5% at 11 weeks, 0% at 12-13 weeks) 2
- For clinically important decisions (such as chorionic villus sampling for sex-linked conditions), delay sex determination until after 12 weeks' gestation 2, 3
Morphologic Variations
- Inaccurate sex determination occurs when external genitalia are malformed 1
- Three-dimensional ultrasound has limited diagnostic value for routine sex determination but may help define congenital malformations of external genitalia 1
Technical Limitations
- Maternal obesity, fetal position, and oligohydramnios can limit visualization 5
- If transabdominal views are inadequate, transvaginal examination can be performed as an adjunct 5