['To determine the frequency of subchorionic hematomas (SCH) in first-trimester ultrasound examinations of patients with infertility and recurrent pregnancy loss (RPL) and in patients from a general obstetric population. To determine if the method of assisted reproduction utilized or the use of anticoagulants, such as heparin and aspirin (ASA), influenced frequency of SCH.', 'Prospective, cohort study.', 'Fertility clinic and general obstetrics clinic.', 'Five hundred and thirty-three women who were pregnant in the first-trimester.', 'Not applicable.', 'Frequencies of subchorionic hematomas in women based on diagnosis, use of anticoagulants, and fertility treatment.', 'SCH were identified in 129/321 (40.2%) in the study group compared to 23/212 (10.9%) in the control group. Fertility diagnosis and the use of heparin did not appear to affect the frequency of SCH in the first trimester; however, SCH occurred at an almost four-fold increase in patients taking ASA compared to those not taking ASA, regardless of fertility diagnosis or method of fertility treatment.'] Are subchorionic hematomas increased in early pregnancy in women taking low-dose aspirin?