Work toward the development of primary strategies for the prevention of preterm birth. When there is evidence of modest effects of multiple causes, interventions that address all of these factors should be considered.
Have designs that are common enough to allow for pooling of data and samples, and consider studying high-risk populations to increase the power of the study.
3. Investigate Racial-Ethnic and Socioeconomic Disparities in the Rates of Preterm Birth
As discussed above, preterm birth rates vary substantially by race and ethnicity. The greatest differences in the rates of preterm birth are between African-American and Asian women. Knowledge can be gained by obtaining an understanding of the differences between groups as well as differences among Asian subgroups. Preterm birth rates also vary by nativity and the duration of residence. In 2003, the preterm birth rate was 13.9 percent for foreign-born African Americans but 18.2 percent for U.S.-born African Americans (CDC, 2005i). It is not known, however, 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. 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 their infants preterm than newcomers who had lived in the United States for 5 years or less (Guendelman and English, 1995).
A number of explanations have been studied, including differences in socioeconomic status (SES), maternal risk behaviors, prenatal care, maternal infection, maternal stress, and genetics. Findings related to SES suggest that the disparities in the rates of preterm birth between African American and white women persist after attempts to adjust for socioeconomic differences (Collins and David, 1997; McGrady et al., 1992; Schoendorf et al., 1992; Shiono et al., 1997). Disparities in preterm birth rates by SES have been well documented not only in the United States (Parker et al., 1994a) but also in other 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 levels of poverty are lower and where the population has universal access to high-quality prenatal and other medical care. Furthermore, socioeconomic disparities are associated with other factors, such as maternal nutrition (Hendler et al., 2005), maternal drug use (Kramer et al., 2000), maternal employment (Mozuekewich et al., 2000), prenatal care (CDC, 2005i), and maternal infection. Given the serious doubt about the effects of prenatal care on reduc-