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ment (Macones et al., 2004; Wang et al., 2002). To date, few studies on preterm birth have untangled this interaction in the context of racial disparities (see Chapter 7). Third, it is not known why foreign-born and U.S.-born women of the same racial descent have such disparate rates of preterm birth, given their supposedly common genetic ancestry. The genetic contributions to racial-ethnic disparities in preterm birth are discussed in greater detail in Chapter 7.

In sum, significant racial-ethnic disparities in preterm birth rates exist in the United States. Racial differences in socioeconomic condition, maternal behaviors (including the use of prenatal care), stress, infection, and genetics cannot fully account for the disparities. More research, perhaps performed by the use of a more integrative approach (Lu and Halfon, 2003; Misra et al., 2003; NRC, 2001), is needed to address this persisting problem. As discussed in the introduction, the greatest difference in rates of preterm birth are between African American and Asian/Pacific Islander women. There could be something learned by understanding those differences as well as differences among Asian subgroups. Although the rates among African American women are strikingly high, they have been decreasing in recent years. In contrast, rates among other racial/ethnic groups have been increasing slightly. Examining these trends or differences may potentially help to shed light on racial and ethnic disparities.

Socioeconomic Condition

Disparities in preterm birth rates by socioeconomic condition have been well documented (Kramer et al., 2000) not only in the United States (Parker et al., 1994a but also in countries such as Canada (Wilkins et al., 1991), Sweden (Koupilova et al., 1998), Finland (Olsen et al., 1995b), Scotland (Sanjose et al., 1991), and Spain (Rodriguez et al., 1995), where the rates of poverty are generally lower than elsewhere in the world and women generally have universal access to high-quality prenatal and other medical care. Although the disparities in preterm birth rates by socioeconomic condition are often closely paralleled (and hence confounded) by disparities by racial and ethnic origin, there are notable exceptions. For example, despite their relative socioeconomic disadvantage, Mexican Americans have preterm birth rates comparable to those of non-Hispanic whites (Table 4-2) (CDC, 2005i).

The reasons for socioeconomic disparities in preterm birth rates are unclear and have been relatively unexplored. A number of factors have been implicated, including maternal nutrition, cigarette smoking, substance use or abuse, work and physical activity, prenatal care, genitourinary tract infection, sexually transmitted diseases, psychological factors, and multiple gestations. A general discussion of these risk factors as they relate to preterm

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