Evidence that fetal membranes are not involved in cortisol metabolism: study of dichorionic twin pregnancies.

11 beta-Hydroxysteroid dehydrogenase (11 beta-HSD, E.C.1.1.1.146) activity has been measured in samples of decidua, choriodecidua, and chorion obtained from five sets of dichorionic twins, with radiolabeled cortisol and cortisone used as substrates. 11 beta-HSD activity was significantly higher in the decidua than in the choriodecidua, in both the oxidative (cortisol to cortisone) are reductive (cortisone to cortisol) directions (activity expressed as percentage conversion): 53.4% +/- 6.7% versus 38.5% +/- 7.4% (p < 0.05) and 21.3% +/- 2.2% versus 15.3% +/- 3.2% (p < 0.025) (mean +/- SEM, n = 10). No measurable activity in either direction was found in the chorion obtained from the wall separating the two amniotic sacs (decidua-free chorion). In both decidua and choriodecidua oxidative activity was higher than reductive activity (p < 0.005 for both). These results demonstrate that 11 beta-HSD activity attributed to the chorion is due to decidual contamination.