Transfer of free and conjugated oxazepam across the human placenta

Six women, 13 to 16 weeks pregnant, and 12 women at 38 to 40 weeks gestation, received oral oxazepam about 12 h before legal abortion, by hysterotomy in the former and before elective caesarean section in the latter group. The concentrations of free and conjugated oxazepam in maternal and fetal plasma were determined by gas-liquid chromatography. In early pregnancy the mean ratio between the plasma concentration of total (free + conjugated) drug in the umbilical cord and a maternal vein was 0.6 whereas in late preganancy the ratio vein was 1.1. Both in early and late pregnancy, the free and glucuronide conjugate of oxazepam were found in the fetus at concentrations which indicated transplacental passage of the parent drug and its metabolite. There was great interindividual variation in the plasma levels both of free and conjugated oxazepam.