ABSTRACT
PIP: For 20 or more years it has been known that a single oral dose of 5 mg mestranol (17alpha-ethinylestradiol-2 methyl ether) is effective for preventing conception in the beagle 95% of the time when administered 1-5 days after mating. This study administered a single oral dose of 4.5, 1.5, or .5 mg on Day 5 after mating to 7 young beagle bitches. All were sacrificed on Days 21-28 after mating, the second trimester of pregnancy in this species. 5 of the 7 animals (those receiving .5 and 1.5 mg) had uteri containing degenerate embryos. In 1 animal receiving 1.5 mg, embryos had already disappeared. 1 bitch receiving 4.5 mg was probably not pregnant. In each instance the embryos were free-floating in the lumen of the uterus due to failure of placental attachment and were either retarded or degenerating. Histologically, the innermost lining of the maternal placenta was disrupted in focal areas with the aseptic lesion containing debris and necrotic cells. Areas between such disrupted areas were lined by the thickened wall of the blastocyst, suggesting resorption of the embryos. Plasma progesterone levels rose during the first trimester according to the pattern for beagles, indicating mestranol had no visible effect on ovarian progesterone production. The tentative conclusion is that mestranol dose not interfere with fertilization or early embryonic development but rather with implantation, resulting in resorption of degenerating embryos.^ieng