In the U.S., women classified as black, African American, and Afro-Carribbean are consistently reported to be at higher risk of preterm delivery (Figure 2). Black women have the highest rates of preterm birth among any racial or ethnic group in the U.S. at 13.4%.7 This high preterm birth rate among black women and the relatively stable discrepancy between black women and other racial and ethnic groups both accounts for the overall high rate of prematurity in the U.S. and contributes to a cycle of reproductive disadvantage with far-reaching social and medical consequences.38
Other maternal demographic characteristics associated with preterm birth include low socioeconomic and educational status, low and high maternal ages, and single marital status.39-41 These may represent markers of access to health care and overall health status in the U.S., but the exact mechanisms by which these maternal demographic characteristics are related to preterm birth are unknown. While the data are somewhat equivocal, overall results from observational studies seem to indicate that stressful working environments, long working hours, and undertaking hard physical labor at work are associated with an increase in preterm birth.42-44
Maternal body mass index (BMI) is also associated with preterm birth rates. A low pre-pregnancy BMI is associated with an increased risk of spontaneous preterm birth, whereas obesity can be protective against preterm birth.45 Obese women have a higher rate of indicated preterm delivery associated with gestational diabetes and pre-eclampsia (Figure 3).46 The potential mechanisms for low BMI to be associated with preterm birth include low levels of micronutrients, reduced blood volume and uterine blood flow, and increased maternal infections.47,48
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