Analyses of recorded data showed that fewer young were born in second litters than in first litters of JU/Fa mice and that the reduction in the size of the second litters occurred only when lactation was concurrent with gestation.
The effect of concurrent lactation on prenatal mortality in second pregnancies was experimentally tested. 46% of lactating females and 7% of non-lactating females, mated in the post-partum oestrus, lost whole litters. Dissections of lactating and non-lactating pregnant females showed that concurrent lactation caused a significant decrease in the number of live embryos. This was the consequence of the extremely high post-implantation mortality—48·3% of implanted embryos—in lactating females. The excess deaths occurred mainly in the `middle' period, i.e. from 5 to 7 days after implantation.
Progesterone was injected into nineteen mated lactating females from the 5th to the 17th day of lactation. The doses given were equivalent to 2·5 mg/day over this period. Whole-litter losses did not occur and the incidence of `middle' post-implantation mortality was reduced to `non-lactating' levels. Implantation, which was markedly delayed in untreated lactating females, occurred 1 or 2 days after the first injection in all but one treated female. It is therefore suggested that concurrent lactation causes progesterone deficiency during pregnancy in JU mice.