Rising plasma levels of 19-nortestosterone throughout pregnancy: determination by radioimmunoassay and validation by gas chromatography-mass spectrometry.

A highly sensitive and specific radioimmunoassay was developed to measure 19-nortestosterone (NT), which allowed us to demonstrate this steroid in the plasma of pregnant women. Plasma NT was detectable throughout gestation, reaching values of 12 to 60 pg/ml in the 3rd trimester. These results were validated by gas chromatography-mass spectrometry. No NT was detectable in the plasma of 12 normal men and 12 nonpregnant women in the mid follicular phase of their cycle (detection limit 4 pg/ml). Our results are compatible with current concepts concerning the possible involvement of 19-norsteroids in an accessory biosynthetic pathway for estrogen in the placenta.