Teratogenicity and embryotoxicity of orally administered lynestrenol in rabbits.

Pregnant Belted Dutch rabbits were administered lynestrenol 17-alpha-ethynyl-oestr-4-en-17-beta-ol) orally on days 6-18 of gestation at doses of 0.1, 0.5, and 2.5 mg/kg/day. On day 29 of gestation the does were killed and autopsied and the fetuses were examined for external, visceral and skeletal abnormalities. Lynestrenol administration produced a statistically significant increase in the number of post-implantation loss (p = 0.05) and in the average per cent of abnormal fetuses per dose group (63%, 66%, and 87% for the medicated group, versus 12% for the placebo group, p = 0.05). None of the doses tested was lethal to the does, but the average weight gain was decreased for the medium and the high dose groups. Abnormalities of the central nervous system and skeletal variants were the most frequent findings in the fetuses.