BackgroundMost prescribed medicines during pregnancy are antibiotics, with unknown effects on a foetus and on the infant's acquired microbiome. This study investigates associations between in utero antibiotic exposure and ear infection trajectories over the first decade of life, hypothesising effects on early or persistent, rather than later-developing, ear infections.MethodsDesign & Participants: The Longitudinal Study of Australian Children (LSAC) birth cohort recruited a

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... tive sample of 5107 infants in 2004. Measures: Mothers reported antibiotic use in pregnancy when a child was 3-21 months old (wave 1), and ongoing problems with ear infection every 2 years spanning ages 0-1 to 10-11 years (waves 1 to 6). Analysis: Latent class models identified ear infection trajectories, and univariable and multivariable multinomial logistic regression determined odds of adverse trajectories by antibiotic exposure. Results4500 (88.1% of original sample) children contributed (mean baseline age 0.7 years; 51.3% boys); 10.4% of mothers reported antibiotic use in pregnancy. Four probability trajectories for ear infection emerged: "consistently low" (86.2%), "moderate to low" (5.6%), "low to moderate" (6.7%) and "consistently high" (1.4%). Antibiotic use in pregnancy was associated with children following "consistently high" (aOR 2.06, 95% CI 1.09 to 3.91, p=0.03) and "moderate to low" (aOR 1.78, 95% CI 1.25 to 2.53, p=0.001) trajectories.ConclusionsAntibiotic use in pregnancy is associated with an increased risk of persistent and early childhood ear infections. This highlights the wisdom of cautious antibiotic use during pregnancy, and the need for study of potential mechanisms underlying these associations.