MINIMAL PROGESTERONE SUPPORT REQUIRED FOR THE MAINTENANCE OF PREGNANCY IN MICE

A study of the degree of progesterone support required for the mainten ance of various stages of pregnancy was undertaken in mice, Mated fema les were ovariectomized at various stages of pregnancy and progesteron e and oestradiol support provided by s.c. Silastic implants with known release characteristics, In the earliest stages of pregnancy (days 1- 5), very low concentrations of progesterone (<25% of normal physiologi cal values) were sufficient to maintain pre-implantation stages and al low implantation., In the immediate post-implantation period (clays 5- 9), the development of implantation sites and decidualization required considerably higher progesterone support, In mid-pregnancy (days 11-1 4), progesterone alone could not maintain pregnancy unless present in very high amounts; however, the presence of oestradiol during this per iod lowered the progesterone requirements to well within the physiolog ical range, This effect of oestradiol started on day 11 but required t he level of oestradiol support to be kept within strictly defined limi ts, with high concentrations inducing abortion, Progesterone alone was able to maintain pregnancy from day 15, These results indicate that t he minimal progesterone support required for pregnancy in mice varies considerably at different stages of pregnancy and is at least partly m odulated by oestradiol.