Nausea and vomiting of pregnancy, "morning sickness", is a common contemporary complaint. Many pregnant patients note alterations in smell and taste which can precipitate "morning sickness", symptoms that characterize early gestation. Epidemiologic studies suggest that pregnancies accompanied by "morning sickness" have better outcomes than asymptomatic pregnancies. The intimate connection between immunogenetic identity, chemoidentity, and chemocommunication by olfactory mechanisms suggests a relationship between maternal symptoms and maternal accommodation of paternal antigens contained in the fetoplacental unit. Most mammalian species utilize olfaction to reduce inbreeding and thus do not require an intimate placental connection between mother and fetus. The evolution of Homo sapiens included prolonged periods of small, genetically homogeneous foraging groups which limited selection of genetically heterogeneous mates. Adaptation to this circumstance included a reduction of olfactory precision in mate selection and a more intimate association between mother and fetus, the hemochorial placenta (Rev Med Chile 2002; 130: 580-84)