Timing of gestation in rats by fetal and maternal weights.

Weights of embryonic and fetal rats increase with gestational age from Day 13 to 20 in a strictly logarithmic manner and then slow. Data for other rat strains published as long ago as 1915 show substantially the same growth curves and the same weights as the inbred Kx strain studied. During this late gestational period fetal weights doubled every 1.22 days. An earlier period of somewhat faster growth can be inferred. These growth rates are virtually the same as the maximum rates at which rat neoplasms can grow. A regression equation was constructed from which the ages of rat embryos can be predicted from their weights within confidence limits of +/- 0.24 days. A similar logarithmic increase of added weight in the pregnant dam was recognized by which pregnancies could be conveniently diagnosed as early as Day 7. These two procedures facilitate the routine production of embryos of known age for experimental purposes.