An Exteroceptive Block to Pregnancy in the Mouse

Abstract

EXPERIMENTS on the effect of certain oral progestogens during early pregnancy, in continuation of previous observations¹ on non-pregnant animals, involved placing a recently mated female receiving oral progestogen with a strange male. In a number of the mice the procedure resulted in failure of pregnancy from the first mating and a new mating within 3–6 days. Control experiments showed that the same effect was produced by dosage with inert material or even without any treatment other than the introduction of a strange male at 24 hr. after mating. 20 out of 49 females behaved in this way, a far greater proportion than could be attributed to the expected incidence of anovular cycles. Only about 8 per cent of young females, as used, return to œstrus within 4–5 days if removed from the male after their first mating, or copulate again at this time if they are left with the male. Moreover, among the suspect females there was a failure of the pseudo-pregnancy which might have been due to poor stud males. Experiments were therefore undertaken to explore this effect.