Background: Prenatal tobacco exposure (maternal active and passive smoking) has been associated with childhood obesity. It is unclear whether this is directly attributable to in utero exposure, with a graded effect across exposure groupings, or to associated lifestyle factors. Methods: In 971 maternal-offspring pairs from the GUSTO cohort, maternal plasma cotinine at 26-28 weeks' gestation and smoking history were used to categorise mothers into prenatal tobacco exposure groups. Offspring anthropometry was obtained longitudinally from birth and at ages 3, 6, 9, 12, 15, 18, 24, 36 months (m). Multiple linear regression, mixed linear models and counterfactual-based analysis were conducted, adjusting for ethnicity, maternal age, education, 1st trimester BMI, parity and gestational age. Findings: All results were compared with unexposed offspring (undetectable maternal cotinine and no history of tobacco exposure). Infants with low maternal cotinine (0·17-13·99 ng/ml) had a similar birthweight, while those with high maternal cotinine (≥14ng/ml) had a lower birthweight [β(95%CI) -162·0g (-291·8,-32·2)]. Low maternal cotinine was associated with higher offspring weight z-score at 18m [0·24 SDs (0·01,0·47)] and 36m [0·27 SDs (0·03,0·52)], and higher BMI z-score at 6m and across 15-36m [36m: 0·45 SDs (0·20,0·70)]. Over the 36m period high maternal cotinine was associated with accelerated gain in weight zscore [0·17 SDs/year (0·03,0·31)]. There was a dose-dependent BMI z-score acceleration, with offspring of mothers with low and high cotinine showing 0·09 (0·01,0·17) SDs/year and 0·17 (0·02,0·32) SDs/year gain, respectively. The association of prenatal tobacco exposure with 36m BMI was not explained by reduced breast-feeding, poor diet and increased screen time. Interpretation: Avoiding even low levels of prenatal tobacco exposure may reduce the risk of early childhood obesity. In utero programming by tobacco exposure directly influences childhood BMI rather than through unhealthy childhood lifestyle factors. Funding Statement: Singapore's National Medical Research Council and Singapore Institute for Clinical Sciences, A*STAR. Declaration of Interests: KMG is supported by the UK Medical Research Council (MC_UU_12011/4), the National Institute for Health Research (NIHR Senior Investigator (NF-SI-0515-10042) and the NIHR Southampton Biomedical Research Centre) and the European Union (Erasmus+ Programme Early Nutrition eAcademy Southeast Asia-573651-EPP-1-2016-1- DE-EPPKA2-CBHE-JP). KMG, Y-SC, and YSL have received reimbursement for speaking at conferences sponsored by companies selling nutritional products. KMG, S-YC and Y-SC are part of an academic consortium that has received research funding from Abbott Nutrition, Nestec and Danone. The other authors have no financial or personal conflict of interest to declare. Ethics Approval Statement: The study was ethically-approved by the Institutional Review Boards of the KK Women’s and Children’s Hospital, and the National University Health System. Informed written consent was obtained from all participants
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