Objectives To study the association between antibiotic intake in pregnancy and the development of otitis media and placement of ventilation tubes (VTs) in the offspring under the hypothesis that antibiotics in pregnancy may alter the offspring's propensity for disease. Study design Data from the 700 children in the Copenhagen Prospective Studies on Asthma in Childhood 2010 unselected birth cohort study were used. Information on maternal antibiotic use and other exposures during pregnancy was collected prospectively from interviews and validated in national registries. Otitis media episodes were registered in a prospective diary for 3 years. Information regarding children's VTs was obtained from national registries. Results There were 514 children who had diary information and were included in the analysis regarding otitis media episodes. For VTs analysis, 699 children were included. Thirty-seven percent of the mothers received antibiotics during pregnancy, and this was associated with increased risk of otitis media (adjusted hazard ratio 1.30; 95% CI 1.04-1.63; P = .02). The risk of receiving VTs was especially associated with third trimester antibiotics (adjusted hazard ratio 1.60; 95% CI 1.08-2.36, P = .02). The risk of otitis media increased with increasing number of treatments (per-level adjusted hazard ratio 1.20; 95% CI 1.04-1.40; P = .02), but for VTs this association was not significant after adjustment. Conclusion Maternal use of antibiotics during pregnancy is associated with an increased risk of otitis media and VT insertions in the offspring. Antibiotics late in pregnancy mainly contributed to these effects, pointing toward potential transmission of an unfavorable microbiome from mother to child.