Objective: We sought to estimate the association between severity of maternal pre-pregnancy underweight and feto-infant morbidity outcomes.
Methods: Missouri maternally linked cohort records from 1989 to 1997 inclusive were analysed. Using pre-pregnancy maternal body mass index (BMI), we classified study participants into: Normal (18.5-24.9) [referent group], mild thinness (17.0-18.5), moderate thinness (16.0-16.9) and severe thinness (<16.0). We estimated the association between pre-pregnancy underweight, underweight subtypes and feto-infant morbidity outcomes using adjusted odds ratios to approximate relative risks with correction for intra-cluster correlations.
Results: Fetal growth curve trajectories for the two groups became divergent as from 30 gestational weeks. Underweight mothers were at increased risk for low birthweight (OR = 1.82; 95% CI = 1.77-1.88), very low birthweight (OR = 1.41; 95% CI = 1.31-1.51), small for gestational age (OR = 1.80; 95% CI = 1.76-1.84), preterm (OR = 1.37; 95% CI = 1.33-1.40) and very preterm (OR = 1.42; 95% CI = 1.34-1.50). These risk estimates increased in a dose-effect fashion with increasing severity of underweight status except for very preterm (p for trend < 0.01).
Conclusion: Pre-pregnancy underweight is a risk factor for a spectrum of feto-infant morbidity outcomes, with risk estimates being most pronounced among extremely underweight mothers.