A subchorionic hematoma, also known as subchorionic hemorrhage or subchorionic bleeding, is when blood forms between the wall of the uterus and the chorionic membrane during pregnancy. The chorionic membrane is the outermost layer separating the babys amniotic sac from the wall of the uterus. It can be associated with vaginal bleeding during pregnancy, which can range from heavy with clotting to light spotting. The specific causes of subchorionic hematoma are not clear, but some risk factors associated with them include malformation of the uterus, history of recurrent miscarriages, history of pelvic infections, trauma, early-onset preeclampsia, severe high blood pressure, and in vitro fertilization. Subchorionic hematomas are most common in people who are between 10 to 20 weeks pregnant and account for approximately 10% of all vaginal bleeding. They can shrink in size and resolve on their own without treatment, but larger ones tend to cause more bleeding and problems. Subchorionic hematomas increase the chance of pregnancy complications, particularly miscarriage and preterm delivery. Women who experience vaginal bleeding during pregnancy should contact their healthcare provider immediately.