Effects on the Course of Pregnancy of Norethynodrel with Mestranol (Enovid) Administered to Rats During Early Pregnancy

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Abstract
Estrogens administered to rats during the first few days post coitum inhibit nidation. Norethynodrel, which has 3-7% the estrogenicity of estrone, was approximately 2% as effective in inhibiting nidation in rats. Both mestranol and norethynodrel with mestranol (Enovid) also prevented nidation in this species. Treatment after the time of nidation did not prevent development of the embryo except in some cases when the dose of estrone was increased approximately 50- to 100-fold. The dose of norethynodrel required to prevent implantation, when administered after ovulation in rats, is approximately 5 times larger than that required to prevent conception when administered in the preovulatory period in the same species.