Using inbred mice with defined major histocompatibility complex (MHC) haplotype antigens housed under specific-pathogen-free conditions for mating and infection allows precise control over potentially important confounding factors, including maternal age, parity, maternal-fetal genetics, and prior pathogen exposures, so that the impacts of pregnancy on E. coli infection susceptibility can be addressed in isolation. The gene discussed is HLA-C; the disease is escherichia coli infection.