We investigated the hypothesis that mode of delivery affects childhood behavior and motor development and examined whether there are sex-specific associations, i.e., whether males and females have different risk estimates.Families with infants born between December 2007 and May 2008 (N = 11,134) were randomly selected and recruited to the Growing Up in Ireland study. Mode of delivery was classified into spontaneous vaginal delivery; instrumental vaginal delivery; emergency Cesarean section (CS); and elective CS. The 'Ages and Stages Questionnaire' was completed at age 9-months and the 'Strengths and Difficulties Questionnaire' at 3 years. Data were weighted to represent the national sample (N = 73,662) and multivariate logistic regression was used for the statistical analyses.At age 9 months, elective CS was associated with a delay in personal social skills [adjusted odds ratio, aOR 1.24; (95 % confidence interval, CI 1.04, 1.48)] and gross motor function [aOR 1.62, (95 % CI 1.34, 1.96)], whereas emergency CS was associated with delayed gross motor function [aOR 1.30, (95 % CI 1.06, 1.59)]. At age 3 years there was no significantly increased risk of an abnormal total SDQ score across all modes of delivery.Children born by elective CS may face a delay in cognitive and motor development at age 9 months. No increase in total SDQ score was found across all modes of delivery. Further investigation is needed to replicate these findings in other populations and explore the potential biological mechanisms.