Mestranol as an abortifacient in the bitch.

Abstract The use of estrogenic compounds for the management of mismating is a widely used technique for birth control in the bitch, although neither is the mechanism of action fully understood, nor is the minimal effective dose of any estrogen used known. A single oral dose of 4.5, 1.5, or 0.5 mg mestranol (17α-ethinylestradiol-2 methyl ether) was given on Day 5 after mating to seven young Beagle bitches. All animals were sacrificed 16–23 days later, on Days 21–28 after mating, and the genital organs processed for histological evaluation. Five of the seven animals had uteri containing degenerate embryos (0.5 and 1.5 mg); in one animal embryos had already disappeared (1.5 mg), and one bitch was probably not pregnant (4.5 mg). All embryos seen were either retarded in development or degenerating; none was implanted. The innermost lining of the maternal placenta was disrupted in focal areas with aseptic and necrotic lesions. Plasma progesterone levels were within expected early metestrus levels. The tentative conclusion was drawn that mestranol does not seem to interfere with progesterone production, fertilization or early phases of embryonic development, but with implantation; the fate of the degenerating embryos is resorption.