Is T-shaped uterine cavity morphology associated with adverse pregnancy outcomes after transfer of a single thawed euploid blastocyst?In this secondary analysis of a prospective cohort study, 648 patients with three-dimensional ultrasound (3D-US) data obtained on the day before embryo transfer were categorized into three groups according to uterine cavity morphology: normal (n = 472), intermediate (n = 166) and T-shaped (n = 10). Quantitative uterine cavity dimensions were used to evaluate uterine cavity morphology. Pregnancy outcomes, including live birth, clinical miscarriage and ectopic pregnancy, were compared among the groups.The prevalence of a T-shaped uterus in this cohort was 1.5%. Uterine cavity morphology was strongly associated with the ratio of interostial distance and isthmic diameter (P < 0.01). Live birth rates were 66.5% for normal, 65.7% for intermediate and 40.0% for T-shaped cavity morphology. Women with a T-shaped uterus had an increased risk of clinical miscarriage (40.0% versus 7.0% for normal and 9.0% for intermediate cavity morphology, P < 0.01) and ectopic pregnancy (10.0% versus 1.1% for normal and 1.9% for intermediate cavity morphology, P = 0.05). When evaluating interostial distance and isthmic diameter ratio to determine pregnancy outcomes, a cut-off value of 2 was noted to have weak predictive value for live birth, but not clinical miscarriage or ectopic pregnancy.T-shaped uterine cavity morphology is associated with adverse pregnancy outcomes after transfer of a single thawed euploid blastocyst. Given the low prevalence of this condition, quantifying the magnitude of risk will require a larger cohort of patients.