Amniotic fluid testosterone and fetal sex determination in the first half of pregnancy.

The concentration of freely extractable testosterone in amniotic fluid was measured in 54 patients whose gestational ages ranged from 10 to 20 weeks; 31 of the fetuses were male and 23 were female. The mean amniotic fluid testosterone concentration was significantly higher with a male fetus than with a female fetus. On only 7 out of 31 occasions was the presence of a male fetus associated with amniotic fluid testosterone values below 100 pg/ml, whereas the values associated with a female fetus were never higher than 90 pg/ml. After 14 weeks gestation no amniotic fluid testosterone level was lower than 120 pg/ml in the presence of a male fetus.