Effects of Age and Parity on Reproduction in Young Female Mice

Summary. The effects of age and of parity on reproductive capacity were estimated by comparing old virgin with young virgin mice, and old parous with old virgin mice, respectively. Increased age was associated with a greater body weight at `joining', a smaller proportion of mice pregnant, a greater number of cl of pregnancy, a greater number of implantation sites at Day 5 or 6 of pregnancy, a greater number of viable fetuses at Day 18 of pregnancy, and a smaller proportion of cl represented as implantation sites at Day 5 or 6 of pregnancy. Parity significantly increased body weight at `joining' and the number of cl of pregnancy. The number of implantation sites at Day 5 or 6 of pregnancy and the number of viable fetuses at Day 18 of pregnancy were non-significantly greater for old parous than old virgin mice. It was concluded from the relationships between body weight at `joining' and the number of cl of pregnancy that the age and parity effects on the numbers of cl, implantation sites and viable fetuses were due to differences between groups in body weight at mating.