Fetal Sex Assignment by Sonographic Evaluation of the Pelvic Organs in the Second and Third Trimesters of Pregnancy

OBJECTIVE: The purpose of this study was to determine the feasibility and reliability of fetal sex assignment by prenatal sonography of the pelvic organs in the second and third trimesters. METHODS: A prospective study, on an axial pelvic image with the umbilical arteries in cross section at the midpoint of the bladder, measured the distance between the posterior wall of the bladder and the anterior wall of the rectum. The bladder interface was scored as concave, flat, or convex. Endometrium visualization was recorded. A video loop of the pelvis was obtained. Each case was read by a second reader aware of only the gestational age. RESULTS: The study population included 205 fetuses. The primary reader was 98.8% accurate in identifying true female fetuses and 100% accurate in identifying true male fetuses. The intraclass correlation coefficient for axial measurement was 0.99. Video loop analysis achieved 96% inter-reader concordance. In the second trimester, measurements in 100% of male fetuses were less than 3.3 mm, and those in 94% of female fetuses were greater than 3.3 mm. In the third trimester, measurements in 96% of male fetuses were less than 4.7 mm, and those in 100% of female fetuses were greater than 4.7 mm. Ninety-eight percent of all fetuses with concave interfaces were female. The endometrium was visualized in 74%. The additional time per examination was less than 5 minutes in 87.7%. CONCLUSIONS: Internal pelvic fetal sex assignment is a reliable additional method for fetal sex determination. A numerical discriminatory level can be used to distinguish between male and female internal genitalia. Larger numbers will be required to further refine these values.