Pregnancy in a Breastfeeding Mouse without Mating

In this report, a rare case of pregnancy in a mouse without mating and during breastfeeding is presented. Out-bred NMRI mice were prepared for mating. Two female mice without superovulation were caged with one male. The next morning, vaginal plaque was not detected; therefore, females were kept in the cage with the same male for 4 more days. One of the females delivered 4 offspring: one female and three males. This female conceived again without mating, 21 days after the first delivery and during the breastfeeding period and this time delivered 16 offspring: 13 males and 3 females. The more likely explanation for this rare, second parturition is embryonic diapause.