Determination of fetal sex.

Fetal sex was determined ultrasonically by visualization of the external genitalia with 99 per cent accuracy in 69 per cent of 855 consecutive fetuses of 15 or more weeks' gestational age. Definitive visualization of the external genitalia was achieved in more than 90 per cent of all cases after the fifth month. We also found that testicular descent into the scrotum occurs after 26 weeks' gestational age, with 62 per cent of male fetuses having descent between 28 and 30 weeks and 93 per cent after 32 weeks. The wide span of gestational age at the time of testicular descent limits use of this feature as an indicator of fetal maturity.