Objective: Recent data show that maternal prepregnancy body mass index (BMI) and gestational weight gain (GWG) are associated with offspring neurobehavior in childhood. However, little is known about the effect on infants that less than 20 months of age, and whether this association has sex differences. Methods: In this birth cohort study, a total of 661 mother–infant pairs were enrolled in Shanghai, China, between February 2017 and April 2019. Maternal prepregnancy BMI was categorized according to the Chinese classification and GWG according to the 2009 Institute of Medicine (IOM). Neurobehavioral development for infants of 12 months of age was assessed by Gesell Developmental Scale (GDS), which contained five subscales of gross motor, fine motor, adaptive behavior, language, and social behavior. Results: Abnormal maternal prepregnancy BMI and excessive GWG were associated with infant birth weight and/or birth length (p < .05), while no influence was found on yearling weight or length. Women who were overweight/obese prior to pregnancy or excessive GWG during pregnancy had infants who were more deficient in neurobehavioral developmental including language (p < .01) and/or social behavior (p < .05). Specifically, excessive GWG was associated with lower language ability in girls but not boys (p < .05). Conclusions: Aberrant prepregnancy BMI and excessive GWG not only affect the body size of newborn infants, but also impair their neurobehavioral development, suggesting that general guidance to the women should be advised to attain optimal prepregnancy BMI and GWG [ABSTRACT FROM AUTHOR]