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J. C. McCARTHY

Summary.

A summary of several estimates of the partial regressions of foetal weight and placental weight at 17½ days gestation on (i) the number of implants in a uterine horn and (ii) the number of implants in a whole litter is presented for litters of one outbred stock and four highly inbred lines of mice. These estimates suggest that the effects of litter size on foetal and placental size are similar in different inbred lines but that (a) the systemic effect of litter size on foetal growth is different and (b) the local effect of implants number on placental growth is different, at least until the 17th day of gestation, in the outbred stock.

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J. C. McCARTHY

Summary.

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.