Superpregnancy in the Mouse: 2. Weight Gain During Pregnancy Available to Purchase

Weight increase of female mice during pregnancy was studied in normal pregnancies, and also in females of various ages in which superovulation and mating had been induced by gonadotrophin treatment. In some spontaneous and induced pregnancies, the weights of the gravid uterus and of the live embryos were taken at autopsy $1812$ days post coitum in addition to the total weight of the intact pregnant female.

The main weight increase in both treated and control pregnancies occurred between about $1012$ and $1612$days post coitum. The weight gain per embryo showed a linear decrease with increasing numbers of embryos, and a linear increase with the $712$-day weight of the female. When the data were adjusted to a standard $712$-day maternal weight, the total weight gain at first rose with embryo number, but declined when the number of embryos increased above the normal range.

The extra-uterine weight increase of the female declined with increasing numbers of embryos. We conclude that female mice carrying abnormally large numbers of embryos economized not only on embryonic growth but also on extra-uterine weight increase. Young mice carrying large litters appeared to use up a large proportion of their fat reserves during pregnancy.