The aim of this study was to determine the association of maternal pre-pregnancy body mass index (BMI) and pregnancy outcome among pregnant women in Croatia. Women who gave birth in Croatia in 2017 were categorised into four groups according to pre-pregnancy BMI and analysed regarding maternal characteristics and perinatal outcomes. Among 32,051 pregnant women, 5.3% were underweight, 65.5% had normal BMI, 20.4% were overweight and 8.8% were obese. Overweight and obese women had a higher prevalence of induction of labour and delivery by caesarean section compared to normal-BMI women (p < .001), while underweight and obese women had a higher prevalence of preterm birth (p < .001). Gestational diabetes (GD), gestational hypertension and preeclampsia were higher in overweight and obese group. Multivariate logistic regression model showed that older maternal age, higher BMI, GD, gestational hypertension and preeclampsia were significant predictors of having non-spontaneous onset of labour and of not delivering vaginally.Impact statementWhat is already known on this subject? Women with abnormal body mass index (BMI) compared to women with normal BMI have an increased risk of various complications during pregnancy and labour such as preterm birth and low birth weight for underweight women, and foetal macrosomia, labour induction and delivery by caesarean section for obese women.What do the results of this study add? This study was performed on a large national cohort of pregnant women and its results show that the majority of adverse perinatal outcomes present in obese women are also present among overweight women. Additional attention should be paid to women with a modest increase in BMI who present the majority of women with abnormal BMI.What are the implications of these findings for clinical practice and/or further research? Risks identified in this study must be acknowledged and taken into account when counselling and managing women before and during pregnancy and labour. Women must be encouraged throughout pregnancy to either gain weight or limit weight gain if they are in adverse BMI group.

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