LITTER SIZE AND THE DURATION OF PREGNANCY IN MICE

¹ Department of Physiology, University of Edinburgh

Analysis of the data from 1246 dated mouse pregnancies revealed an inverse relationship between litter size and gestation period which, however, was not continuously linear over the range of litter sizes as previously shown [Biggers et al., 1963], but was mostly evident as an increase in mean duration with litters below modal size. An additional cause of this effect is suggested.

The effect on pregnancy of the administration of stradiol in amounts likely to cover the ’physiological’ range was investigated. No reduction in gestation period was observed; with the higher doses, pregnancy was significantly prolonged. The findings do not support the hypothesis that the litter size - gestation period effect involves placental strogen.