The concentration of progesterone, 20 alpha-dihydroprogesterone, testosterone, oestrone and oestradiol-17 beta in serum, amniotic fluid and placental tissue of pregnant rabbits.

The concentrations of progesterone (delta 4P), 20 alpha-dihydroprogesterone (20 alpha-DHP), testosterone (T), oestrone (E1) and oestradiol-17 beta (E2 beta) in peripheral blood serum (PBS), amniotic fluid (AF) and placental tissue of rabbits during gestation were determined by radioimmunoassay. The placenta of the 10-day pregnant rabbit was fragile and composed mainly of maternal tissue. By the 12th day of pregnancy it was separable into maternal and foetal placentae. The mean concentration of delta 4P in PBS rose from 200 pg/ml (day 1 pregnancy) to 17--21 ng/ml (days 10--15) and decreased gradually to 1 ng/ml a few hours before parturition. The 20 alpha-DHP in PBS also showed an increase from 1.5 ng/mg (day 1) to 12 ng/ml (day 6) but fluctuated thereafter. The concentration of 20 alpha-DHP in the PBS tended to be lower than that of delta 4 P during pregnancy until the regression of the corpus luteum. An interesting observation was an increase of T on days 6--8 of pregnancy, the time when implantation occurs. The concentrations of E1 and E2 beta in PBS remained very low throughout pregnancy. delta 4P and 20 alpha-DHP in AF ranged between 25 pg to 1 ng/ml and in no case during the course of pregnancy were the levels of T, E1 and E2 beta in AF higher than in PBS. Where the maternal placental delta 4P content remained between 1--2 ng/placenta, the foetal placenta delta 4P rose to a level of 15 ng/placenta by day 31 of pregnancy. A similar trend was recorded for 20 alpha-DHP content. It is concluded that although a parallelism between PBS and myometrial steroid concentration was observed, no relationship could be drawn between the concentrations of steroid in PBS and those of the placental tissue and AF.