Background Several potential early-life risk factors for childhood overweight and obesity have been identified. Yet results regarding early risk factors and obesity (BMI-for-age ≥30 kg/m2) at pre-school age are mixed and boys and girls have rarely been studied separately. Our aim was to study whether pre- and perinatal factors predict obesity in 5-year-old boys and girls. Methods National register data was used to identify children born in Finland between 2007 and 2014 with data available on

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... - and perinatal factors as well as on height and weight at 5 years of age (n = 131,818). Multivariable logistic regression models were used to analyze sex-specific associations between pre- and perinatal factors (e.g. maternal age, parity, pre-pregnancy BMI, smoking, diabetes, delivery method, gestational-age-adjusted birth weight) and offspring obesity. Results Five percent of boys (n = 3,551) and 3% of girls (n = 1,949) had obesity. Adjusted for potential confounders, the strongest predictor of subsequent obesity at 5 years of age was severe maternal obesity (BMI ≥35 kg/m2) (boys: OR 6.5, 95% CI 5.8-7.4; girls: OR 7.5, 95% CI 6.4-8.8) followed by maternal obesity (BMI 30.0-34.9 kg/m²) (boys: OR 3.9, 95% CI 3.5-4.4; girls: OR 5.5, 95% CI 4.8-6.2) and maternal overweight (BMI 25.0-29.9 kg/m²) (boys: OR 2.3, 95% CI 2.1-2.5; girls: OR 2.6, 95% CI 2.3-3.0). Other factors associated with child obesity were maternal smoking during pregnancy (boys: OR 1.8, 95% CI 1.6-1.9; girls: OR 2.0, 95% CI 1.8-2.2) and being born large for gestational age (boys: OR 1.9, 95% CI 1.6-2.2; girls: OR 2.1, 95% CI 1.8-2.6). Conclusions These findings indicate that childhood obesity may partly originate before conception, gestation and birth. As prevention of obesity may be economically and socially more sustainable than treatment of its consequences, emphasis should be put on early interventions. Particular attention should be paid on preventing maternal overweight and obesity. Key messages Maternal obesity was the strongest predictor of child obesity at 5 years of age. Investing in maternal health from as early as before conception may help prevent offspring obesity.