Minimal progesterone support required for the maintenance of pregnancy in mice

A study of the degree of progesterone support required for the maintenanceof various stages of pregnancy was undertaken in mice. Mated females wereovariectomized at various stages of pregnancy and progesterone andoestradiol support provided by s.c. Silastic implants with known releasecharacteristics. In the earliest stages of pregnancy (days 1-5), very lowconcentrations of progesterone (<25% of normal physiological values)were sufficient to maintain pre-implantation stages and allow implantation.In the immediate post-implantation period (days 5-9), the development ofimplantation sites and decidualization required considerably higherprogesterone support. In mid-pregnancy (days 11-14), progesterone alonecould not maintain pregnancy unless present in very high amounts; however,the presence of oestradiol during this period lowered the progesteronerequirements to well within the physiological range. This effect ofoestradiol started on day 11 but required the level of oestradiol supportto be kept within strictly defined limits, with high concentrationsinducing abortion. Progesterone alone was able to maintain pregnancy fromday 15. These results indicate that the minimal progesterone supportrequired for pregnancy in mice varies considerably at different stages ofpregnancy and is at least partly modulated by oestradiol.