If you experience vaginal bleeding early in pregnancy, your doctor might order a uterine ultrasound to determine the source. In some cases, vaginal bleeding is associated with a partial separation of the placenta from the uterine wall. A blood clot, or hematoma, forms in the uterus at the bleeding site. Blood clots in the uterus can vary in size and location. Clots can develop at the edge, behind or in front of the placenta. The location and size of the clot can affect the degree of risk it poses to your pregnancy.
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Types of Hematomas
If your doctor finds a blood clot in the uterus on ultrasound, he might diagnose you with a hematoma or hemorrhage. This complication affects approximately 3 percent of all pregnancies in the first trimester, a July 2003 "Obstetrics and Gynecology" study reports. Hematomas are classified by their location, relative to the placenta. Subchorionic hematomas occur along the edge of the placenta. Those that occur behind the placenta are called retroplacental hematomas. Amniotic hematomas aren't associated with placental separation, but with rupture of blood vessels near the umbilical cord. They occur in front of the placenta and are very rare. In the July 2003 "Obstetrics and Gynecology" study, 57 percent of hematomas were subchorionic, and 43 percent retroplacental.
Why Blood Clots Occur
In most cases, there is no known reason why part of the placenta separates from the uterine wall, resulting in the formation of a blood clot in the uterus. If you have a blood clotting disorder, you may have a higher risk of bleeding and forming a clot. Maternal abdominal injury from an automobile accident or other trauma may cause placental separation followed by bleeding and clot formation. High blood pressure and cocaine use are also risk factors for a uterine hematoma caused by placental separation. Amniotic hematomas most often occur during labor and delivery, from traction on the umbilical cord.
The presence of a hematoma can increase the risk of pregnancy complications, such as preterm delivery and pregnancy loss. However, complications don't always occur. A meta-analysis of studies published in the August 2012 "Proceedings in Obstetrics and Gynecology" found a threefold increase in placental abruption -- premature separation of the placenta from the uterine wall after 20 weeks.
Placental abruption accounts for 25 percent of all fetal deaths, a March 2001 article in "American Journal of Roentgenology" states. Pregnancy-induced hypertension occurred twice as frequently in women with subchorionic hematomas. Preterm delivery occurred twice as often in women with subchorionic or retroplacental hematomas. Increase in complications such as an abnormal heart rate during labor and poor fetal growth was also noted in the 2003 "Obstetrics and Gynecology" study.
However, a study presented at the 2013 World Congress of the International Society on Ultrasound in Obstetrics and Gynecology found no increase in miscarriage in women with subchorionic hematomas, the most common type of hematoma.
If you have a uterine hematoma, it will usually resolve without treatment. In some cases, a small hematoma might not be diagnosed until you deliver. Your doctor might recommend reduced activity and restricted travel if a subchorionic clot develops before 20 weeks. You might need frequent ultrasounds to assess the size of the clot and watch for complications if you have any type of hematoma. Report any vaginal bleeding to your medical practitioner immediately.