Pre-weaning mortality was low in all groups and unaffected by dam genotype. One wild type female had 1 pup stillborn and 2 born alive but lost between day 1 and cross fostering. Two acasein deficient females lost 2 pups and one pup respectively between day 1 and cross fostering. No pups were lost from cross fostering until day 21, but on day 22 two pups were found dead in the largest litter (11 pups before day 22) nursed by an a-casein deficient female.