Failure of Transient Progesterone Deprivation to Induce Ova Implantation in Ovariectomized Rats.

Summary Daily injections of 4 mg progesterone caused a delay in implantation of ova in 50 rats ovariectomized on the third day of pregnancy. The day sperm were found in the vaginal smear was designated day 1 of pregnancy. This response was not altered by temporary withdrawal of progesterone on day 9, 9 and 10, or 13 of pregnancy. Nidation was subsequently induced in the same animals shortly after beginning daily injections of 1 μg estrone in combination with 4 mg progesterone. The number of rats with implantation sites and total number of sites in the experimental groups was not significantly different from corresponding control values. At autopsy 16 days after beginning estrone-progesterone treatment, embryo survival was significantly lower only when progesterone was deprived on day 9 of pregnancy.