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Objective: To assess the association of first-trimester subchorionic hematomas with pregnancy outcomes after 20 weeks of gestation in women with singleton pregnancies.Methods: We conducted a retrospective cohort study of all women with singleton pregnancies presenting for prenatal care before 14 weeks of gestation over a 3-year period at a single obstetric practice. All patients underwent routine first-trimester ultrasound examinations. We compared rates of adverse pregnancy outcomes at more than 20 weeks of gestation in women with and without a subchorionic hematoma on the initial ultrasound examination, excluding women with pregnancy loss before 20 weeks of gestation.Results: From January 2015 to December 2017, a total of 2,172 women met the inclusion criteria for the study, 389 (17.9%) of whom had a subchorionic hematoma (mean largest diameter 2.1±1.4 cm). Women with subchorionic hematomas had their first ultrasound examination at earlier gestational ages (8 5/7 vs 9 6/7 weeks, P<.001) and were more likely to have vaginal bleeding at the time of the ultrasound examination (31.9% vs 7.9%, P<.001). Maternal age, race, use of in vitro fertilization, body mass index, and medical comorbidities did not differ between the groups. On univariable analysis, subchorionic hematoma was not associated with any pregnancy outcomes at more than 20 weeks of gestation, including gestational age at delivery, preterm birth, birth weight, birth weight less than the 10th percentile for gestational age, gestational hypertension, preeclampsia, placental abruption, intrauterine fetal death at more than 20 weeks of gestation, cesarean delivery, blood transfusion, and antepartum admissions. On regression analysis including subchorionic hematoma, vaginal bleeding, and gestational age at ultrasound examination, vaginal bleeding was independently associated with preterm birth at less than 37 weeks of gestation and birth weight less than the 10th percentile. Subchorionic hematoma was not independently associated with pregnancy outcomes. This study had 80% power to detect a 5% absolute difference in the frequency of preterm birth; that is, from 10% to 15%.Conclusion: In singleton pregnancies, a first-trimester subchorionic hematoma is not associated with adverse pregnancy outcomes at more than 20 weeks of gestation. [ABSTRACT FROM AUTHOR]