preterm birth, that some sociodemographic factors that are associated with preterm birth are also associated with some birth defects, or that the two conditions may share other maternal risk factors or medical conditions (Rasmussen et al., 2001). Further investigation is needed to understand this association.
Low maternal prepregnancy weight and body mass index (BMI) have consistently been associated with preterm birth. After adjusting for confounders (previous preterm labor, previous low birth weight, standing at work >2 hours, abruptio placentae, urinary tract infection and stress score >5), Moutquin (2003) noted that women with BMIs of less than 20 were nearly four times as likely as heavier women to have a spontaneous preterm birth (OR 3.96; 95% CI 2.61–7.09). Indeed, the relationship between low prepregnancy BMI and spontaneous preterm birth is consistent (OR 1.7– 3.9) among North American caucasians (Moutquin, 2003), blacks (Johnson et al., 1994), and urban Latinas (Siega-Riz et al., 1994, 1996).
A low BMI also modifies the contribution of low pregnancy weight gain to the risk of preterm birth (Schieve et al., 2000). Compared with normal-weight women (BMI 19.8–26.0) with adequate weight gain during pregnancy (0.5–1.5 kg/week), the risk of spontaneous preterm birth at less than 37 weeks of gestation is sixfold greater for underweight women (BMI < 19.8) with poor pregnancy weight gain (< 0.5 kg/week) and threefold greater for normal-weight women with poor pregnancy weight gain.
A randomized trial of treatment for bacterial vaginosis (BV) examined the incidence of preterm delivery in at-risk women, including those who were underweight (Hauth et al., 1995). Women who had a previous spontaneous preterm birth or a prepregnancy weight of less than 50 kilograms (N = 624) were randomly assigned to metronidazole and erythromycin antibiotic therapy or placebo. Among the 258 women with BV, a lower rate of preterm delivery in those assigned to the treatment group was observed for women who weighed less than 50 kilograms (N = 81). The incidence of preterm birth was 33 percent in the placebo group and 14 percent in the antibiotic-treated group (p = 0.04). This is in contrast to the findings of several investigators who demonstrated that treatment of BV in a general obstetric population is ineffective (Carey et al., 2000).
In a cohort of more than 70 percent white women, Sebire and colleagues (2001) noted a decreased frequency of delivery at less than 32 weeks of gestation among women with BMIs greater than or equal to 30 (OR