Introduction International studies on this topic yield controversial results. No studies from Russia have been published. We contribute by studying associations between maternal pre-pregnant BMI and the risk of spontaneous preterm birth in Northwest Russia.

Methods A historical cohort study using the data of the Murmansk County Birth Registry. All spontaneous singleton births in the county in 2006–2008 were included (n=23 527). All women were categorised as underweight (<18.5 kg/m²), normal (18.5–24.9 kg/m²), overweight (25.0–29.9 kg/m²), and obese (≥30.0 kg/m²). Crude and adjusted associations between pre-pregnant BMI and risk of spontaneous preterm birth (<37 completed weeks) were estimated by logistic regression analyses.

Results In total, 7.5% (95% CI 7.2 to 7.9) of all spontaneous births were preterm. The prevalence of preterm birth according to the mother's BMI was 7.3% (95% CI 6.9 to 7.7) for normal weight, 7.6% (95% CI 6.4 to 8.9) for underweight, 8.2% (95% CI 7.4 to 9.1) for overweight and 8.3% (95% CI 7.0 to 9.6) for obese women. The risk of spontaneous preterm birth was increased in overweight women compare to normal weight mothers (crude OR 1.15, 95% CI 1.13 to 1.31). After adjustment for marital status and education, increased risks of preterm delivery were found in both overweight (OR 1.18, 95% CI 1.04 to 1.34) and obese women (OR 1.21, 95% CI 1.00 to 1.45). Further adjustment for age, parity, infant sex, smoking and alcohol abuse, reduced the associations to non-significant level.

Conclusion Overweight and obesity are associated with preterm birth in Nothwest Russia, but only before adjustment for biological factors. More detailed analyses of causal pathways are warranted. Potential mechanisms will be discussed.