A short umbilical cord is associated with adverse pregnancy outcomes. However, there is no universally accepted definition of a short cord. Objective: This study aimed to determine the umbilical cord length showing the highest correlation with adverse pregnancy outcomes. Materials and Methods: The authors retrospectively analyzed the clinical data of women who attempted vaginal birth in the present institution. Umbilical cord lengths were categorized into three groups: less than the first percentile, from the first percentile to less than the tenth percentile, and others. Maternal and neonatal characteristics previously suggested to affect cord length were evaluated. The main outcome was the rate of cesarean delivery. The authors also evaluated the frequency of operative vaginal delivery, small-forgestational-age (SGA) births, neonatal intensive care unit (NICU) admission, umbilical artery pH < 7.1, and abnormal bleeding during delivery. Results: Cord lengths of 35 and 45 cm corresponded to the first and tenth percentiles, respectively. A short cord was an indicator of unplanned cesarean delivery and small-for-gestational-age births. Conclusion: An umbilical cord length of ≤ 45 cm is a clinically useful indicator of adverse pregnancy outcomes.