Abstract
Vaginal bleeding in the first trimester is common, but our understanding of how bleeding relates to the risk of pregnancy loss is limited. In a prospective cohort study, participants provided information on bleeding episodes during early pregnancy and underwent a first-trimester research ultrasonogram. Among 5,425 participants, 25.0% of participants reported first-trimester bleeding, and 12.0% of pregnancies ended in loss. Bleeding was not associated with risk of pregnancy loss (hazard ratio [HR] 0.89, 95% CI, 0.75-1.07). First-trimester bleeding coupled with ultrasound measurements that lagged more than 5 days behind last menstrual period (LMP) dating was associated with increased risk (HR 2.72, 95% CI, 2.10-3.53). These findings indicate that reassurance is appropriate for most patients experiencing first-trimester bleeding when ultrasound measurements align with LMP.