Objective

Extremes of prepregnancy maternal BMI increase neonatal mortality and morbidity at term. They also increase the risk of extremely preterm (EP, i.e., <27 weeks' gestational age) births. However, the association between maternal BMI and outcomes for EP babies is poorly understood.

Methods

We used a cross-country design, bringing together the following three population-based, prospective, national EP birth cohorts: EXPRESS (Sweden, 2004–2007); EPICure 2 (UK, 2006); and EPIPAGE 2 (France, 2011). We included all singleton births at 22 to 26 weeks' gestational age with a live fetus at maternal hospital admission. Our exposure was maternal prepregnancy BMI, i.e., underweight, reference, overweight, or obesity. Odds ratios (OR) for survival without severe neonatal morbidity to hospital discharge according to maternal BMI were calculated using logistic regression.

Results

A total of 1396 babies were born to mothers in the reference group, 140 to those with underweight, 719 to those with overweight, 556 to those with obesity, and 445 to those with missing BMI information. There was no difference in survival without major neonatal morbidity (reference, 22%; underweight, 26%, OR, 1.31, 95% CI: 0.82–2.08; overweight, 23%, OR, 1.00, 95% CI: 0.77–1.29; obesity, 19%, OR, 0.94, 95% CI: 0.70–1.25).

Conclusions

No associations were seen between maternal BMI and outcomes for EP babies.