37.3 percent of infants born at 32 to 36 weeks gestation were delivered by cesarean.

The rise in cesarean rates corresponds to a rise in maternal age. In 2003, the cesarean delivery rate for women 25 to 29 was 26.4 percent. For 35-39 year old women, the rate was 36.8 percent. This may be related to increased multiple births for older women, biological factors, or patient-practitioner concerns (Ecker et al., 2001; CDC, 2005i). Some findings suggest relatively low risks for maternal morbidity (for example, anesthesiarelated complications, infection) (Bloom et al., 2005; Lynch et al., 2003), while others suggest that there are increased risks such as readmission in the postpartum, infection, and complications related to anesthesia, among others (Koroukian, 2004; Hager et al., 2004; Liu et al., 2005; Lumley, 2003).

Dramatic demographic changes in the late 20th century have resulted in increased levels of education and rates of employment among women, including the employment of married women and mothers of young children. The U.S. Department of Labor (DOL, 2005) reported that in 2003 more than half of married American women and more than half of mothers of young children were employed, with the most highly educated women being the most likely to be working. The current proportion of women in the civilian workforce (56 percent) presents a 54 percent change from the proportion in 1980 (DOL, 2004). There has been an increasing trend for women to delay childbearing into their 30s. In 2003, women ages 30 to 34 experienced the highest birth rate for women in this age group since the mid-1970s and women ages 40 to 44 had the highest birth rate for women in this age group since the late 1960s (CDC, 2005a). For women ages 35 to 39, the birth rate increased 47 percent between 1990 and 2003, although the increase in the population of women aged 35 to 39 was only 7 percent (CDC, 2004a, 2005a) (Figure 1-5). A slight increase in the birth rate among women between the ages of 25 and 29 was observed in 2003. In contrast, over the past decade, the birth rates among adolescents (ages 15 to 19) and women ages 20 to 24 have decreased. The current birth rate among adolescents is the lowest rate recorded for this age group in the United States (CDC, 2005a).

Women age 35 and older have increased rates of preterm birth (see Figure 4-1 for discussion and also see Appendix B). Although adolescents also have increased rates of preterm delivery, the numbers of births among women in this age group, as noted above, have decreased in the last decade. Older mothers are more likely to have underlying medical conditions, such as diabetes and hypertension. The rates of these diagnoses reported in birth certificate data have risen since the early 1990s, in tandem with the rise in