Placental transfer of fentanyl in early human pregnancy and its detection in fetal brain.

We have investigated the transfer of fentanyl across the early human placenta in 38 women (8-14 weeks' gestation) undergoing termination of pregnancy. After administration of a bolus dose of fentanyl 2 micrograms kg-1 at induction of anaesthesia, maternal blood n = 38), placenta (n = 38), amniotic fluid (n = 38) and fetal brain (n = 7) samples were collected and assayed for fentanyl by radioimmunoassay. Fentanyl was detected in all placental and fetal brain samples but not in amniotic fluid. There was a rapid decrease in fentanyl concentrations in maternal serum after the bolus but placental concentrations had not started to decline 30 min later. There was no difference in placental drug concentrations at different gestational ages. These data suggest that there is rapid transfer of fentanyl to the fetus in early pregnancy and that the drug remains in fetal tissue for some time after the initial dose is given to the mother.