Animal procedures

BALB/c mice (Charles River), C57BL/6J mice, and placenta-specific P0 knockout mice (P0 mice) with deletion of the U2 exon within the Igf2 gene were housed, and all procedures were performed according to procedures approved by the Animal Research Committees at University of California, Santa Barbara, or in accordance with the UK Animals (Scientific Procedures) Act 1986 at the University of Manchester. Animals had free access to food and water and were maintained on a 12-hour light/12-hour dark cycle at 21° to 23°C. Following mating, the presence of a copulation plug was denoted as embryonic day 0.5 (E0.5) of pregnancy. P0 mice were originally a gift from M. Constância (Babraham Institute, UK). Heterozygous males with the P0 deletion were mated with C57BL/6J female mice (wild type; 6 to 10 weeks of age, which resulted in mixed litters consisting of both wild-type and P0 fetuses) (25).