eral studies have examined the relationship between cohabitation outside of marriage and preterm birth (Blondel and Zuber, 1988; Manderbacka et al., 1992). A collaborative case-control study in 16 European countries found that a significantly elevated risk of preterm birth was associated with cohabitation compared with the risk for those who are married (although the odds of preterm birth were substantially lower for women who cohabit with their partners than for single mothers) in countries where less than 20 percent of births occur outside of marriage. In contrast, no excess risk was associated with marital status in countries where out-of-marriage births were more common (Zeitlin et al., 2002). A population-based study in Quebec (where 44 percent of births were to common-law mothers in 1997) found the preterm birth rate among mothers in common-law unions to be higher than that among mothers in traditional marriage relationships but still lower than that among unmarried mothers living alone (Luo et al., 2004). The adjusted odds ratios for preterm birth were 1.14 and 1.41 for common-law and single mothers living alone, respectively, when individualand community-level characteristics were controlled for. A large hospital cohort study in Finland also found a higher risk of preterm birth among single women than married women (adjusted odds ratio = 1.29), with the risk of cohabiting women being somewhere in between (adjusted odds ratios = 1.15) (Raatikainen et al., 2005).

Preterm birth rates vary substantially by race and ethnicity in the United States (CDC, 2005i). As discussed in Chapter 1, there are significant interand intragroup variations in the risk of premature birth. For example, in 2003 the preterm birth rates among Hispanics ranged from 11.7 percent for Mexicans to 13.8 percent for Puerto Ricans. Preterm birth rates also varied by nativity and duration of residence. In 2003, the preterm birth rate was 13.9 percent for foreign-born blacks but 18.2 percent for U.S.-born African Americans (CDC, 2005i). Even the duration of residence seems to have an effect on preterm birth rates. A study in California found that long-term Mexican immigrants who had lived in the United States for more than 5 years were more likely to deliver preterm infants than newcomers who had lived in the United States for 5 years or less (Guendelman and English, 1995). In general, the literature examining potential causes of racial and ethnic disparities in preterm births is not well developed. Investigations of the effects of nativity are even less developed.

These racial-ethnic disparities have persisted for decades, although the precise reasons are not clear. A number of explanations have been proposed, including racial differences in socioeconomic condition, maternal