Embryotoxic effects of post-coital fertility inhibiting steroids

In mice post-coital estrogens caused retardation of tubal egg cleavage, whereas post-coital progestins stimulated egg division. In rats the situation was reversed. In this species estrogens accelerated the tubal embryo development and vice versa; the progestin STS 557 arrested division of embryos resting in the tube. Normal implantation occured following transfer of early blastocysts from estrogen-treated mice to untreated mice, but not following transfer of blastocysts from untreated animals to estrogen-treated animals. This shows that post-coital estrogens do not affect the embryos directly but they disturb the physiological integrity of the endometrium required for proper implantation.