interval on low birth weight was explained by an increased prevalence of underweight among women with short interpregnancy intervals (Ferraz et al., 1988). In the general obstetric population at low risk for adverse pregnancy outcomes, Adams et al. (1997) found that short interpregnancy intervals are rare and are weak risk factors among low-risk women, and thus, efforts to lengthen interpregnancy intervals are unlikely to reduce substantially the rates of adverse pregnancy outcomes among these women. Smith et al. (2003) estimated the attributable risk fractions of a short interpregnancy interval of less than 6 months to preterm birth at 24 to 32 weeks and 33 to 36 weeks of gestation to be 6 and 4 percent, respectively. In a largely African American cohort of women, Blackmore-Prince et al. (2000) found that the median interpregnancy interval was 15 months (range, 1 to 207 months), with 19 (4 percent) of the women having interpregnancy intervals of less than 3 months. After adjustment for parity, gestational age (in weeks), and smoking status, the mean birth weight associated with an interpregnancy interval of 3 or more months was 3,106 grams, 215 grams greater than that for an interpregnancy interval of less than 3 months (p = 0.06).

Infertility treatments have allowed thousands of couples who have difficulty conceiving to fulfill their desire to have children. In the United States in 2002, 7.3 million women, or 12 percent of women ages 15 to 44, had physical difficulty becoming pregnant or carrying a baby to term. Approximately 2.1 million of these women, or 7 percent of all women between the ages of 15 and 44, were infertile, defined as not becoming pregnant after 12 months when the couple is not using contraception (CDC, 2002b). Two percent of women had had an infertility-related medical appointment within the previous year, and an additional 10 percent reported that they had received services for infertility at some point in their lives.

The use of infertility treatments has risen dramatically in the past 20 years and has been associated with the trend to delay childbearing (see Chapter 1). In 2002, 33,000 American women delivered babies as a result of the use of infertility procedures; this is more than twice the number who had done so in 1996 (Meis et al., 1998). More than 50 percent of these women were 35 years of age or older. In recent years, an unintended consequence of the use of these technologies, multiple gestations and the increased risk for preterm delivery, has become a focus of attention. There is also evidence that a portion of the reported association between infertility treatments and preterm birth may be attributable to the underlying biological reasons for infertility and subfecundity (long time to becoming pregnant) (Basso and Baird, 2003; Henriksen et al, 1997; Joffe and Li, 1994).