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. © 1959 Nature Publishing Group.