Immune cells fight infection and cancer. Surprisingly, maternal immune cells in the womb help fetuses to grow (see https://science.sciencemag.org/content/365/6456/862.abstract). The environment in the womb marks the progeny for life, programming their health and predisposition to disease. Babies born too small, for example, may develop cardiovascular disease later in life. We seek to understand how the signals triggered by interactions between genetic variants of immune system genes influence the biology of both special uterine immune cells called natural killer (NK) cells and of placental cells and, in turn, pregnancy outcomes. For example, we are determining how NK cells and their “education” through interactions of immune system genes, impact on uterine vascular changes, placental development and fetal growth. These interactions are part of a complex network involving many cells and molecules. We are starting to crack this immunological code of pregnancy and may one day intervene to improve outcomes.