There is growing concern about the potential effect of parental obesity on the newborn child's birth outcomes and health. However, few studies have examined whether parental obesity before pregnancy affects the risk of adverse pregnancy outcomes. We aimed to systematically evaluate the associations of adverse pregnancy outcomes with prepregnancy body-mass index (BMI) of the mother and the father. The China National Free Preconception Health Examination Project (NFPHEP) was a nationwide project beginning in 2010 across China to assess the risk factors for birth defects and other adverse pregnancy outcomes. The cohort included couples living in rural areas who were in the reproductive age and planning pregnancies within 4–6 months. Data on weight, height, and BMI status before pregnancy and birth outcomes were obtained from women and their partners aged 20–49 years who had pregnancies within 1 year after baseline examinations between 2010 and 2015 in selected 220 counties of 31 provinces. Multivariate logistic regression models were used to estimate odds ratios (ORs) and 95% CIs with adjustment for covariates. The current study was approved by the Institutional Research Review Board at the National Health and Family Planning Commission and National Research Institute for Health and Family Planning, and all participants gave written consent. The cohort included 1 501 557 women and 1 463 597 men. We included undated data with complete baseline and follow-up data in the cohort. Participants included women and their husband or single women, thus the cohort contained more women than men. We analysed independent association for mothers and fathers using the separate samples, and analysed the combined association using men's samples. Compared with women with normal prepregnancy BMI (18.5–23·9 kg/m2, 74%), the following outcomes were more common in women who were overweight (24–27·9 kg/m2, 10%) or obese women (≥28·0 kg/m2, 2%): preterm birth (OR 1·11; 95% CI 1·09–1·14 vs 1·22; 1·17–1·29), low birth weight (1·11; 1·05–1·18 vs 1·36; 1·22–1·52), macrosomia (1·38; 1·33–1·42 vs 1·65; 1·54–1·74), stillbirth (1·17; 1·04–1·32 vs 1·42; 1·14–1·77), neonatal mortality (1·22; 1·07–1·40 vs 1·31; 1·00–1·70), and birth defects (1·20; 1·02–1·41 vs 1·07; 0·76–1·51). The corresponding ORs for men with high BMI (overweight 24–27·9 kg/m2 [22%]; obese ≥28·0 kg/m2 [5%]) versus men with normal BMI were as follows: preterm birth (1·12; 1·10–1·14 vs 1·24; 1·20–1·28), low birth weight (1·10; 1·05–1·15 vs 1·29; 1·20–1·40), macrosomia (1·19; 1·16–1·22 vs 1·34; 1·28–1·40), stillbirth (1·12; 1·02–1·23 vs 1·19; 1·00–1·41), and birth defects (1·12; 0·99–1·28 vs 1·32; 1·05–1·64). Additionally, couples in which both partners had BMI greater than 24 kg/m2 (5%) had higher odds of adverse pregnancy outcomes than couples with normal BMI. The findings from this large cohort of Chinese couples of reproductive age show that increasing pre-pregnancy maternal and paternal BMI, both independently and combined, increases the risk of adverse pregnancy outcomes such as preterm birth, low weight birth, macrosomia, stillbirth and birth defects. The National Key Research and Development Program of China (2016YFC1000102, 2016YFC1000307, 2016YFC1000307-6), National Natural Science Foundation of China (81602854), National Scientific Data Sharing Platform for Population and Health (2016NCMIZX06).