Subchorionic Hematoma and Pregnancy Risks

Bleeding during pregnancy can be alarming but isn't always a sign of an impending miscarriage. Sometimes bleeding is a symptom of a fairly common type of blood clot called a subchorionic hematoma that's formed by the abnormal accumulation of blood between the placenta and the wall of the uterus. Why this happens isn't entirely understood, but there are several possible causes.

And although there's nothing that can be done about a subchorionic hematoma, in many cases treatments wouldn't be necessary even if it existed. Under most circumstances, women who develop subchorionic hematomas enjoy otherwise normal pregnancies and go on to have perfectly healthy, happy babies.

The causes of subchorionic hematoma aren't fully understood. It's thought that in some cases, bleeding may occur when the placenta—the organ that forms during pregnancy to provide oxygen and nutrients to the developing fetus and also to carry away waste—dislodges fully or partially from the uterine wall. Another possibility is that when the fertilized egg implants into the uterine wall the attachment is abnormal in some way.

There are some suspected risk factors for subchorionic hematomas. Studies have found they're more likely to occur in pregnancies that are conceived via in vitro fertilization (IVF) or frozen-thawed embryo transfer, for example. Women who've already had one baby or who are older also appear to be more likely to develop subchorionic hematomas.

Bleeding caused by a subchorionic hematoma can range from a heavy flow with clots to light spotting to no bleeding at all, in which case the only reason the clot is discovered is that it shows up during an ultrasound. Vaginal bleeding is estimated to affect as many as one in four women during the first half of a pregnancy and is a common reason for first-trimester ultrasonography. Some women also have some cramping, especially if the bleeding is significant.

Hematomas found during the early part of the first trimester tend to be less problematic than those discovered later in the first or second trimester. And generally speaking, small hematomas on the surface of the placenta are far less concerning than those that develop under the placenta or behind the fetal membrane.

Hematomas that don't grow also are less worrisome, but those that do may cause the placenta to pull away from its attachment site on the uterus. If more than 30 percent of the placenta becomes dislodged, it could cause the hematoma to grow even larger. This may set off a domino effect in which the membranes (amniotic sac) ruptures prematurely, leading to a spontaneous abortion.

In fact, research has found that subchorionic hematoma can increase the risk of an array of pregnancy complications, including miscarriage, preterm labor, placental abruption, and premature rupture of membranes. The risk is largely related to the size of the hematoma, how far along the pregnancy is, and the mother's age.

Once a subchorionic hematoma forms there's nothing a doctor can do about it: There's no treatment for these blood clots. However, they can be managed. If you develop one, depending on the location and size of the hematoma, your doctor may advise you to come in for regular follow-up exams. He may also have you avoid strenuous activity, heavy lifting, or excessive exercise. You may need frequent rest to prevent increases in blood pressure. Staying well hydrated can help prevent constipation and the subsequent straining that might set off bleeding.

In rare cases, a doctor may recommend blood thinners to bleed the clot out. If the chances of miscarriage are high, some doctor will use estrogen and progesterone therapy to slow or prevent further hemorrhaging.