Effects of Parity and Litter Size on the Birth Weight of Inbred Mice

Fifty-four litters of C57BL/6 mice born to mothers of various parities were weighed within 15 hours of birth, their litter sizes recorded and the mean weight per individual calculated. Regression curves of birth weight on litter size were drawn separately for infants born to primiparous and to multiparous mothers. The birth weights of the infants were found to be related to both the parity of their mothers and the size of their litters. Individuals born to mothers that had previously experienced parturition weighed significantly more at birth than those born to previously nulliparous mothers. Independently of maternal parity, individuals from smaller litters weighed significantly more at birth than those from larger litters.