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; 95% CI: 3.96; 2.61-7.09). Indeed, the relationship between low prepregnancy BMI and spontaneous preterm birth is consistent (odds ratios = 1.7 to 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 six fold 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) supports these data (Hauth et al., 1995). Women with BV and a prepregnancy weight of less than 50 kilograms were randomly assigned to metronidazole and erythromycin antibiotic therapy or placebo. The incidence of preterm birth was 33 percent in the placebo group and 14 percent in the antibiotic-treated group (p = 0.04). Thus, treatment of BV in underweight women reduces the risk of preterm birth. 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).
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