Metabolism of 14C-pregnenolone in the placenta throughout pregnancy in organ culture.

The present study was undertaken to assess the metabolism of radiolabeled pregnenolone in the placenta throughout pregnancy in organ culture. The product generated from pregnenolone was only progesterone regardless of the weeks of gestation. The amount of progesterone formed by the placenta was 5.60 +/- 0.15 x 10(-11) moles/mg protein/hour in the 1st trimester (n = 4), 6.47 +/- 0.09 x 10(-11) moles/mg protein/hour in the 2nd trimester (n = 3) and 7.72 +/- 0.25 x 10(-11) moles/mg protein/hour in the 3rd trimester (n = 6). The increase in the formation of progesterone as gestation advanced was statistically significant. The increase in the placental weight throughout pregnancy was in proportion to the rise in maternal plasma levels of progesterone. These data indicate that the manifold increase in progesterone in the maternal circulation as gestation advances mainly reflects the increase in the functional mass of the placenta rather than the increased rate of production of progesterone in the placenta.