An analysis of 143 litters of Peromyscus polionotus showed that litter size increased by 0·30 mice/litter up to the fourth pregnancy. After the fourth litter, litter size increased at a lesser rate to the sixth pregnancy.
Growth followed the same general pattern as did litter size. The effect of order of litter was significant for weights up to 28 days of age. In general, average weight of the mice increased up to the sixth pregnancy and then decreased.