Introduction The birth of a large for gestational age (LGA) infant is a significant risk factor for birth complications and maternal morbidity and an even higher risk factor for offspring obesity, metabolic syndrome and cardiovascular disease in later life. Relevant factors affecting the risk of delivering an LGA infant are maternal pre-gravid obesity, excessive gestational weight gain exceeding the recommendations of the Institute of Medicine (IOM) and diabetes in pregnancy. We aimed to determine what matters most in terms of the risk of fetal overgrowth. Materials and Methods We performed a database analysis of 12 701 singleton term deliveries documented in our university hospital birth registry from 2003 to 2014. Multivariate logistic regression analysis was used to determine the adjusted odds ratios. Results Excessive weight gain had the strongest impact on LGA (OR: 1.249 [95% CI: 1.018 - 1.533]) compared to maternal pre-gravid body mass index (BMI) (OR: 1.083 [95% CI: 1.066 - 1.099]) and diabetes (OR: 1.315 [95% CI: 0.997 - 1.734]). Keeping gestational weight gain within the recommendations of the IOM resulted in a risk reduction for LGA of 20% (OR: 0.801 [95% CI: 0.652 - 0.982]). The risk for LGA increases by 6.9% with each kg weight gain. Normal weight women (BMI 18.5 - 24.9 kg/m ² ) and moderately overweight women (BMI 25 - 29.9 kg/m ² ) showed the highest increase in LGA rates per kg weight gain during pregnancy (OR: 1.078 [95% CI: 1.052 - 1.104] and OR: 1.058 [95% CI: 1.026 - 1.09], resp.). Only in underweight (< 18.5 kg/m ² ) and normal weight women the risk of LGA birth is strongly influenced by diabetes (OR 11.818 [95% CI: 1.156-120.782] and 1.564 [95% CI: 1.013-2.415]). Conclusion Excessive weight gain is particularly important for non-obese women. These women are therefore a target cohort for intervention, as each prevented additional kilogram weight gain reduces the risk of LGA by more than 5%.

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