EFFECTS OF CONCURRENT LACTATION ON LITTER SIZE AND PRENATAL MORTALITY IN AN INBRED STRAIN OF MICE

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Abstract
Analyses of recorded data showed that fewer young were born in 2nd litters than in 1st litters of JU/Fa mice and that the reduction in the size of the 2nd litters occurred only when lactation was concurrent with gestation. The effect of concurrent lactation on prenatal mortality in 2nd pregnancies was experimentally tested. Forty six percent 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[long dash]48.3% of implanted embryos[long dash]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 19 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 1 treated female. It is therefore suggested that concurrent lactation causes progesterone deficiency during pregnancy in JU mice.