The role of the pituitary-ovarian axis in the termination of early pregnancy in rabbits treated with ethinyl-oestradiol.

The pathogenesis of embryonic death following maternal treatment with ethinyloestradiol was investigated. Pregnant rabbits received daily oral doses of 0.4 mg/kg body weight of ethinyloestradiol starting on day 6 of pregnancy. Dams were autopsied after 3, 6, or 9 days of treatment. Uterine swellings, ovaries, and the pituitary gland were examined using histochemical techniques. The results showed that in the treated animals there were morphological signs indicating decreased secretory activity of gonadotrophin and prolactin producing cells of the pituitary gland, luteolysis, and embryonic death within the first few days of treatment. The present study indicated that termination of early pregnancy in rabbits is most probably due to the interference of ethinyloestradiol with the pituitary-ovarian axis which in turn results in luteolysis and impairment of luteal steroid synthesis.