Selection. Rural-to-urban migrants are more likely than people who remain in rural areas to have socioeconomic characteristics that in most countries are associated with interest in controlling one's own fertility (42, 201). Rural people who choose to migrate often do so because they want to change their lives for the better. Controlling their own fertility can be an important part of this change. Also, large families may find it harder to move than small families.

Disruption. In itself, the act of migrating typically reduces fertility for a time because it delays marriage, separates spouses, and postpones childbearing (114, 201, 303). Spousal separations of two years or more reduce the number of children that a woman has over her lifetime (234). Some new migrants, however, may still have more children than they would like because they are not aware of family planning as an option (241), do not know about reproductive health services, or lack access to them (40, 43, 107). Disruption is an especially important force in the case of refugees and displaced persons. Typically, fertility of refugees drops in the first six months after flight and then begins to climb steadily (280).

Adaptation. As migrants settle into urban areas, new influences and new social networks change their lives. Like their diets, jobs, and interests, people's fertility preferences, family planning practices, and other reproductive behavior change as they become integrated into urban life (344). Migrants tend to be innovative and flexible (19, 33, 158). Thus it may not take them long to learn about hospitals, clinics, and other urban reproductive health care services (344).

Urban life erodes the social underpinnings of high fertility, which are much stronger in rural areas. For example, in rural Africa supports for high fertility include polygamous marriages, the high social status associated with bearing many children, a woman's obligation to her husband's parents to bear children, and the use of child labor in agriculture. These supports are weaker in urban areas, where monogamy is usually the norm, it costs more to raise children, and children do not make as much economic contribution to the family (42).

Adaptation to urban norms may affect a woman's fertility and her child's health if she no longer breastfeeds a newborn child. Breastfeeding tends to postpone the next pregnancy as well as to provide good nutrition to the infant children. While rural-to-urban migrants as a group breastfeed longer than do urban nonmigrants (43, 272), the duration of breastfeeding falls as migrants' length of residence increases (112, 326).

Family planning programs may need particularly to reach out to the most recent migrants, who are least likely to have adapted to urban life. Most migrants adopt urban fertility norms within a few years, and their fertility rate falls (42, 175). Studies in selected sub-Saharan African cities and a few cities elsewhere have found that, after one or two years of residence in an urban area, migrants from rural areas use contraception more (42, 106,178).

Temporary migrants. People who migrate to urban areas for only a short time often do not fit the selection-disruption-adaptation pattern. Some newly arrived migrants plan to return home soon, while others are ambivalent about the future, waiting to see how things work out in their new place of residence. Still others, known as "target migrants," come to cities only to achieve a particular goal, such as getting an education or earning a certain sum of money (140).

The fact that temporary migrants do not settle in any one place for long makes it difficult for social service programs to reach them (158, 331). Experience in Sri Lanka, however, indicates that, if family planning services are available, transient migrants often do become interested in using them to avoid unwanted pregnancies (215).

Diminishing differences. In some developing countries rural residents are becoming more like urban residents as the ideas and amenities of urban areas spread to rural areas. Radio and television now reach rural areas as well as cities (303). Differences between cities and the countryside in mortality, fertility, and educational attainment have diminished (140). Although the gaps between rural and urban life may be narrowing, there are still important differences in reproductive health needs and behavior between rural-to-urban migrants and urban nonmigrants.