RESUMO
BACKGROUND: Maternal body mass index (BMI) outside the normal range and smoking are both associated with adverse perinatal outcomes, but their interaction needs further investigation. AIM: The aim of this study was to analyse the combined effects of smoking and BMI on birth weight, preterm birth rate, the somatic development of neonates, and complications of pregnancy. MATERIAL AND METHODS: Data from 508 926 singleton pregnancies from the German Perinatal Survey of 1998-2000 were analysed according to maternal BMI and smoking. RESULTS: Preterm birth rates were higher for non-smoking underweight (8.3%) and obese women (6.7%) than for normal weight (6.0%) or overweight women (5.6%); rates were higher in smokers than in non-smokers for every BMI category. The mean birth weight increased with increasing BMI and was decreased by smoking; it was 2,964 g in underweight smokers and 3,556 g in obese non-smokers. Small for gestational age (SGA) rates were least in obese women and highest in underweight women; large for gestational age (LGA) rates varied in the opposite direction. In smokers SGA rates were higher than in non-smokers for every BMI category and LGA rates were always lower. Hypertension, proteinuria, oedema, and pre-eclampsia/eclampsia were more common as BMI increased but were always lower in smokers. Pre-eclampsia/eclampsia occurred in 0.7% of underweight smokers but in 9.6% of obese non-smokers. CONCLUSIONS: Smoking and low maternal BMI in combination can cause high rates of preterm birth and SGA neonates as well as low mean birth weight. Although smoking offers some apparent benefit regarding LGA rates and pre-eclampsia this should not distract from its overall adverse influence.