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Villous tissue from 26 placentae of 7-17 weeks was incubated with radioactive pregnenolone alone and with pregnenolone in the presence of progesterone and 9 synthetic gestagenic steroids and the progesterone formation was measured after 30 min. When progesterone was present in a concentration of 31 or 310 mumol/1 the conversion rate of labelled pregnenolone to progesterone was reduced to 88.6 and 82.2% of that of the respective control incubations. Dydrogesterone, allyloestrenol, lynoestrenol and norethynodrel under similar conditions did not inhibit the formation of progesterone. The inhibitory effects of megoestrol acetate, medroxyprogesterone acetate and norgestrel were close to that of progesterone. Norethisterone and methyloestrenolone were the most effective inhibitors of progesterone formation: when incubated in an equimolar concentration (35 mumol/1) with pregnenolone (50 microgram) the progesterone formation was reduced to 60.0-62.7% and 29.1-34.0% respectively of that of the respective control experiments.

The effect of synthetic gestagens on progesterone formation in vitro in human placenta of early pregnancy