Pregnancy Block: Imitation by Administered Gonadotropin1,2

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Abstract
A normal ovulating dose of pregnant mare serum gonadotropin (PMSG) administered to mice on the first or second day after mating completely blocked pregnancy. When it was administered on the third day the number of implantations was reduced and no effect was observed following treatment on the fourth day. The median effective dose for BALB/ cWt and four-way cross animals was 0.6 IU and for C57BL/10Wt females it was 1.8 IU. The PMSG treatment induced behavioral estrus, increased the rate of egg transport through the oviduct, and reduced the decidual response to sesame oil injected into the uterine lumen. Ten IU of PMSG administered to rats on the day after finding sperm in the vagina also blocked pregnancies. The mechanism by which PMSG blocks pregnancy and its relation to the "Bruce effect" are discussed.

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