Influence of fetal sex on the concentration of amniotic fluid testosterone: antenatal sex determination?

Amniotic fluid testosterone was measured by radioimmunoassay in 226 samples taken between the 12th week of pregnancy and birth. The effect of fetal sex on the level of this androgen was determined and the usefulness of the parameter for predicting fetal sex assessed. Testosterone was measured using an antibody raised in rabbits against testosterone-3-carboxymethyloxime-BSA. The antiserum cross-reacted strongly with 5alpha-dihydrotestosterone so that the values reported were refered to as measuring "immunoreactive testosterone" levels. Male fetal values were highest between the 12th and 15th week of pregnancy reaching a maximum mean of 501 +/- 185 pg/ml gradually falling during the remainder of pregnancy to 281 +/- 74 pg/ml, the difference in values between the first half and the end of pregnancy being highly significant (p less than 0.0005). Female fetal values did not vary significantly during the course of gestation, the minimum mean value being 179 +/- 50 and maximum 253 +/- 98 pg/ml. The fetal amniotic fluid sex difference was highly significant during the first half of pregnancy (p less than 0.0005) and at 12-15 weeks a successful prediction of fetal sex was possible, using testosterone, in 39 out of 41 cases. At birth on the other hand there was no difference between male and female fetal fluid levels. The method can also be used to detect disturbances of intrauterine sexual development as exemplified by the case of a Klinefelter syndrome where female testosterone values were found at the time of sexual differentiation. Thus the measurement of unconjugated testosterone in amniotic fluid would seem to offer not only an addition to chromosomal analysis for prediction of fetal gender but also be a valuable tool for detection of disturbances of intrauterine sexual development i.e. when genetic sex and sex-specific hormone production are discrepant.