We found that a unicornuate uterus is associated with a lower live-birth rate and a higher risk of ectopic pregnancy, miscarriage, preterm delivery and malpresentation at birth in comparison to pregnancies in which the uterus is normally formed. Women with a unicornuate uterus were more likely to have adenomyosis and endometriosis, the risk of the latter being greatest in those with a functional rudimentary horn. What are the clinical implications of this work?These findings should inform the counseling of women with a unicornuate uterus. Removal of a functional rudimentary horn to prevent ectopic pregnancy should be discussed with the patient. Pregnant women with a unicornuate uterus have a higher risk of adverse outcome and warrant close monitoring.