AbstractBackgroundWe examine whether associations between prenatal exposure to hazardous maternal alcohol consumption during the first trimester of pregnancy and sleep problems in young children represent a causal association.MethodsThe population‐based sample consists of 15,911 mothers with 30,395 offspring from the Norwegian Mother, Father, and Child Cohort Study (MoBa) and the Medical Birth Registry of Norway (MBRN). Women self‐reported pre‐pregnancy alcohol consumption and consumption during the first trimester of pregnancy twice: at gestational weeks 17 and 30. Mothers reported their children's sleep problems, when they were 1.5 and 3 years (mean = 50; SD = 10). We tested models adjusting for (1) measured confounders, (2) unmeasured familial risk factors by sibling design, and (3) maternal hazardous drinking in the 3 months prior to pregnancy as an instrumental variable within the sibling design.ResultsChildren of mothers with hazardous drinking during the first trimester were at increased risk of sleep problems at 1.5 (β= 1.14, 95%CI 0.04–2.25) and 3 (β= 2.86, 95%CI 1.85–3.87) years of age. These associations were reduced to close to zero and non‐significant at 1.5 (β= −0.32, 95%CI −1.91–1.26) and 3 (β= 0.06, 95%CI −1.56–1.64) years when controlling for both familial and measured environmental risk factors.ConclusionsThere is a moderate association between maternal hazardous drinking during pregnancy and offspring sleep problems up to age three. This association is explained by risk factors differing between families and does not reflect a cause‐effect relationship.