An increasing body of research indicates that seminal fluid serves as more than simply sperm support media. Seminal fluid exposure at the time of intercourse enhances fertility and placental health. We tested the hypothesis that extended exposure to paternal seminal fluid prior to pregnancy will provide additional fertility benefits, due to the female reproductive tract being primed to male antigens in seminal fluid for a prolonged period of time. Females were primed by housing with vasectomised males for two months prior to pregnancy. Females previously housed with vasectomised males had more embryos in late pregnancy than females that had never mated, and altered placental morphology, with a significant decrease in the size of the labyrinthine zone. We have further tested whether the benefits of earlier seminal fluid exposure requires females to become pregnant with embryos from males that match the genetic and immune profile of the earlier mate. It has been theorised that the maternal immune system can be primed to recognise a male’s major histocompatibility complex (MHC) antigens via ongoing seminal fluid contact, and thereby provide a better fetal/maternal (placental) relationship when the embryo presents the same antigen. While our initial study used the same male MHC for seminal fluid priming and for pregnancy, we have new data to present on the effects of switching to a novel MHC male for pregnancy, and how this impacts subsequent litter size and offspring development.