Background The association between body mass index (BMI) and IVF cycle outcomes remain inconclusive. In addition, the impact of BMI on perinatal outcomes has been less well-studied. The aim of this study was to assess the effects of BMI on pregnancy outcomes, as well as maternal and neonatal outcomes. Methods This was a retrospective cohort study on 10,252 frozen-thawed cycles with single blastocyst transfer between January 2016 and December 2019. Patients were divided into four groups: underweight (< 18.5 kg/m(2)), normal-weight (18.5-24 kg/m(2)), overweight (24-28 kg/m(2)), and obesity (>= 28 kg/m(2)), according to the Chinese classification. Multivariate logistic regression and multivariate general linear model were used for statistical analysis. Results The rates of live birth and clinical pregnancy were comparable among groups. Miscarriage rate was higher in the obese women than that in the normal controls (27.51 vs. 20.91%, aOR = 1.453 (1.066-1.982)). Using the normal-weight women as reference, the underweight women had lower incidences of preterm birth (6.97 vs. 11.19%, aOR = 0.611 (0.422-0.884)), macrosomia (4.90 vs. 8.65%, aOR = 0.544 (0.353-0.837)) and large-for-gestational age (LGA, 11.18 vs. 16.54%, aOR = 0.643 (0.477-0.866)); the overweight women had higher prevalence of gestational diabetes (6.56 vs. 3.82%, aOR = 1.744 (1.232-2.468)), hypertension (4.42 vs. 2.32%, aOR = 1.822 (1.186-2.800)), macrosomia (12.93 vs. 8.65%, aOR = 1.596 (1.240-2.054)) and LGA (23.22 vs. 16.54%, aOR = 1.549 (1.270-1.890)); the obese women had higher incidences of preterm birth (16.87 vs. 11.19%, aOR = 1.646 (1.068-2.536)), cesarean delivery (93.98 vs. 87.91%, aOR = 2.078 (1.083-3.987)), gestational hypertension (4.82 vs. 2.32%, aOR = 2.138 (1.005-4.547)), macrosomia (14.88 vs. 8.65%, aOR = 1.880 (1.192-2.964)) and LGA (25.60 vs. 16.54%, aOR = 1.764 (1.218-2.555)). Conclusions BMI has no significant effect on the chance of pregnancy or live birth, but obesity increases the risk of miscarriage. Underweight is associated with better maternal and neonatal outcomes, while overweight and obesity are associated with worse maternal and neonatal outcomes.