Objective <p>Thyroid hormones are involved in the regulation of haemostasis, and it has been hypothesised that abnormal levels of thyroid hormones are associated with an increased risk of excessive bleeding. However, this association is yet to be investigated in pregnant women.</p> Methods <p>We performed a retrospective nationwide cohort study of Danish pregnant women from 1999 to 2015. Information on maternal hyperthyroidism and hypothyroidism diagnosed before/during or after the pregnancy, as well as outcomes of bleeding in early and late pregnancy, was collected via the Danish nationwide health registers on hospital diagnoses and drug prescriptions. Multivariate logistic regression was used to calculate adjusted odds ratio (aOR) with a 95% confidence interval (95% CI) for the association between maternal thyroid disease and outcomes of bleeding.</p> Results <p>The study cohort included 1,149,871 pregnancies. Maternal hyperthyroidism was diagnosed in 17,702 pregnancies and hypothyroidism in 27,674 pregnancies. Maternal hyperthyroidism diagnosed before/during (aOR 1.11 (95% CI 1.02;1.21)) and after the pregnancy (aOR 1.11 (95% CI 1.01;1.22)) as well as hypothyroidism diagnosed before/during (aOR 1.11 (95% CI 1.03;1.19)) and after the pregnancy (aOR 1.10 (95% CI 1.02;1.18)) were associated with an increased risk of bleeding in the early pregnancy. For bleeding diagnosed in late pregnancy, we found an association only when maternal hypothyroidism was first diagnosed after the pregnancy (aOR 1.22 (95% CI: 1.07;1.40)).</p> Conclusion <p>In a large, nationwide cohort, maternal hyperthyroidism and hypothyroidism showed associations with diagnoses of bleeding in pregnancy. Results provide clues on the role of undiagnosed and untreated maternal thyroid disease in pregnancy.</p> Trial registration <p>Not applicable.</p>