In sub-Saharan Africa, women consider ideal birth spacing to be 2 to 5 years. Sexual relations while lactating are taboo. A new pregnancy during lactation brings criticism to the mother. Public opinion upholds birth spacing as a means to preserve the health of mother and child, especially among the Yoruba of Nigeria where postpartum abstinence may last up to 3 years. It has likely decreased the fertilty of the Yoruba by 25% and will remain the major means to control their fertility. Tribal names for the disease of a breast feeding child whose mother has had sexual intercourse or is pregnant exist in many parts of sub-Saharan Africa (e.g., apa, from the Ikale of Nigeria). Folklore blames sperm for tainting mother's milk which causes kwashiorkor and marasmus in the suckling child. Neighbors associate infant diarrhea, which occurs most often when mothers give supplementary food or when they are weaning a child, with resumption of sexual intercourse. A long postpartum sexual taboo is not universal throughout sub-Saharan Africa, however. For example, postpartum abstinence ranges from 2.5 to 3.9 months in the Sudan, while it ranges from 12.4 months to 30.9 months in Ghana. Abstinence duration of 40 days or less is most common among Moslems, one of 40 days to 1 year is typical in East Africa, and one longer than a year tends to be confined to non-Moslems of West and Central Africa. Postpartum abstinence occurs in gerontocratic societies and in patrilocal and patrilinear marriages outside of one's own group. It serves also as a means of social control by maintaining a distance between spouses. Modernization, urbanization, education, and the influence of Islam are chipping away at the tradition of postpartum abstinence, however, resulting in growing fertility. Every 2.5 months of abstinence prolongs the nonsusceptible period by only about 1 month, mainly because most abstinence occurs during lactational amenorrhea when women are unlikely to conceive.