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FIGURE 4-1 Relationship between maternal age and preterm birth, by race, 1998 to 2000, U.S. birth cohorts.

SOURCE: NCHS (unpublished data).

MARITAL STATUS AND COHABITATION

Pregnancy in unmarried women has been associated with a higher risk of preterm birth (Blondel and Zuber, 1988; Holt et al., 1997; Luo et al., 2004; Olsen et al., 1995b; Peacock et al., 1995; Raatikainen et al., 2005; Wen et al., 1990; Zeitlin et al., 2002). The rates of preterm birth for unmarried and married women were determined by using NCHS data for the 1998 to 2000 U.S. birth cohorts. As shown in Table 4-1, preterm birth rates are higher for unmarried women than for married women across all racialethnic and age groups.

The reasons for the higher rates of preterm birth among unmarried mothers are not known but are commonly attributed to their relative lack of social support and resources (Raatikainen et al., 2005; Waldron et al., 1996). The protective effects of marital status, however, are not uniform across racial-ethnic and age groups; the data in Table 4-1 suggest that marital status appears to offer the greatest protection against preterm birth among older (35 years of age or older) women and non-Hispanic African Americans.

Approximately 40 percent of births that occur outside of marriage now occur to cohabiting couples (CDC, 2000), and the rates of cohabitation have been increasing in the United States within the last few decades. Sev-



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