Extended sexual cohabitation affects pregnancy success. The immunological challenges of pregnancy may have favored long-term mating in humans. Fetal tissue shares only 50% of its genetic’ material with the mother and is often recognized as a foreign body by the mother’s immune system and thus subject to attack. One result of immunological attack is preeclampsia, a severe form of gestational hypertension that occurs in about 10% of all human pregnancies. In extreme cases it may lead to epileptic seizures and possibly result in the death of the fetus (Martin, 2003). Because the risk of preeclampsia usually decreases with subsequent pregnancies, it has long been thought to be a risk only of first pregnancies. However, when studies examined the effect of different fathers on the risk of preeclampsia, it was found that regardless of the overall number of children borne, the critical predictor of, preeclampsia was whether it was a woman’s first child with that partiular father. Women who became pregnant with a new partner experienced the same risk of preeclampsia as women who were in their first ever pregnancy (Dekker & Robillard, 2003; Robillard, Dekker, & Hulsey, 1999). The risk of preeclampsia was also correlated with the duration of the sexual relationship with the father. Women who conceived for the first time within the first 4 months of sexual cohabitation had a 40% risk of preeclampsia, whereas those who conceived after a year ofsexual cohabitation had only a 5% risk (Robillard et aI., 1994). Lengthy sexual cohabitation may not only reduce the risk of disease, but may also have positive effects on the fetus. When conceptions occurred after a year of sexual cohabitation, pregnancies were marked by increased fetal weight relative to placental weight, an indication of fetal health (Robillard et al., 1994). This evidence suggests that the mother’s immune system may learn over time not to reject the genetic material of a long-term sexual partner (Robertson, Bromfield, & Tremellen, 2003; Robertson & Sharkey, 2001) and suggests an additional benefit of forming long-term sexual relationships.