Immunological tolerance to the fetus is essential for fetal survival during pregnancy. The semi-allogeneic fetus expresses genes foreign to the mother that can be recognized by maternal T cells. Under times of stress or infection, deleterious immune responses can result in fetal destruction and/or maternal death. Exposure to non-maternal antigens begins as early as insemination and some of the mechanisms required to prevent maternal priming against these antigens are in place before sexual encounter. Continuous and overlapping regulatory mechanisms must cooperate to allow the best chances for fertilization, implantation, and healthy gestation, simultaneously protecting the fetus from maternal immune attack yet making minimal compromises in resistance to infection. Several types of immune cell from both the innate and adaptive arms of the immune system help protect both the mother and fetus during pregnancy. It's the intricate communication and interplay between the immune system and the endocrine system that will ultimately decide the success or fate of the developing fetus.