16 years of age at the time of their last menstrual period), especially those of young gynecological age (within 2 years of menarche), had a twofold greater risk for preterm delivery compared with the risk for older women (ages 18 to 29 years). Using U.S. natality data, Branum and Schoendorf (2005) also found a nearly twofold greater risk of very preterm delivery (less than 33 weeks gestation) among young adolescents (16 years of age or younger) compared with that among young adults (ages 21 to 24 years); the risk decreased with an increase in the age of the adolescent mothers. It is not known at present whether the increased risk of preterm birth among young adolescents is due to their biological immaturity or to an increased prevalence of other risk factors associated with their generally poor socioeconomic condition (Branum and Schoendorf, 2005; Mitchell and Bracken, 1990; Olausson et al., 2001; Scholl et al., 1992).

Women ages 35 and over are also at increased risk for preterm delivery (Astolfi and Zonta, 2002; Cnattingius et al., 1992). Astolfi and Zonta (2002) found in a population sample of Italian women a 64 percent increased odds of preterm delivery among mothers 35 years of age or older compared with that among mother less than 35 years of age when education, birth order, and fetal gender were controlled for. The risk was particularly striking among mothers over 35 years of age delivering their first-born child. The reasons for the increased risk for preterm delivery among older women are not known. By using pooled data for the 1998 to 2000 U.S. birth cohorts from the National Center for Health Statistics (NCHS), the committee identified a similar U-shaped curve that characterizes the relationship between maternal age and preterm delivery (Figure 4-1).

As shown in Figure 4-1, the association between maternal age and the risk of preterm birth is not consistent across racial and ethnic groups. It is observed that the preterm birth rate begins to rise at a younger age for non-Hispanic African Americans (ages 27 to 29) than for non-Hispanic whites (ages 33 to 35), and the slope of the rise with increasing age is greater for African Americans than for whites. Geronimus (1996) attributes this differential rise with increasing age to “weathering.” According to the “weathering” hypothesis, the effects of social inequality on health compound with age, leading to growing gaps in health status between African American and white women through young and middle adulthood that can affect their reproductive outcomes. However, evidence supporting the weathering hypothesis remains inconclusive, as most studies that use cross-sectional data cannot adequately control for potential cohort effects. Further studies on the interaction effects of maternal age and race-ethnicity on preterm birth are needed.