Marchut M
Experientia. 1976 Sep 15;32(9):1211-3. doi: 10.1007/BF01927631.
4-14C-Progesterone and 4-14C-pregnenolone are metabolized in vitro by rabbit placenta, at day 15 and 28 of gestation, exclusively to compounds reduced in ring A (5beta) and at carbon 3 and 20.