One out of every three pregnancies has blood loss in the first trimester of pregnancy. It may be caused by a minor problem or a symptom that something is wrong. One of the most common causes of bleeding during early pregnancy is intrauterine hematomas.
These are crescent or wedge-shaped structures between the chorionic tissue and the uterine wall or fetal membranes (between the tissue that is created for the pregnancy and the more superficial layers of the uterus). They appear when small pools of blood occur within the endometrium, between these layers.