Hematomas during pregnancy most often form when the chorion, the outer membrane surrounding the embryo, partially separates from the uterine wall. Blood collects in that gap, creating what’s called a subchorionic hematoma. It shows up on somewhere between 1.7% and 18% of pregnancies overall, making it one of the most common findings on first-trimester ultrasounds. The exact trigger for the separation isn’t fully understood, but several risk factors make it more likely.
How a Subchorionic Hematoma Forms
During early pregnancy, the fertilized egg implants into the uterine lining and the chorion develops as a protective outer layer. Normally, this membrane stays firmly attached to the decidua, the specialized lining of the uterus. A subchorionic hematoma results from a partial detachment of the chorionic membrane from the decidual membrane. Once that separation occurs, blood from small maternal blood vessels pools in the space between them.
The hematoma itself is simply a pocket of trapped blood. In many cases it stays small, gets reabsorbed by the body, and causes no problems. Some women notice vaginal bleeding or spotting, which is often how the hematoma is discovered. Others have no symptoms at all, and the hematoma only appears on a routine ultrasound.
Known Risk Factors
While no single cause has been pinpointed, certain conditions raise the odds significantly.
Autoimmune conditions are among the clearest risk factors, particularly antiphospholipid syndrome. This condition causes the immune system to produce antibodies that interfere with normal blood flow in the placenta, making the chorionic membrane more vulnerable to separating from the uterine wall.
Blood clotting disorders (thrombophilias) also increase risk. When clotting doesn’t function normally, even minor disruptions at the implantation site can lead to bleeding that accumulates rather than resolving on its own.
IVF and assisted reproduction carry a notably higher rate. One study found subchorionic hematomas in 22.4% of IVF pregnancies compared to 11% in naturally conceived pregnancies. Within IVF, specific factors pushed the risk even higher: frozen embryo transfers carried about six times the odds, and blastocyst transfers nearly four times the odds compared to other IVF approaches. The hormonal stimulation involved in IVF cycles and the high estrogen levels that result may alter the uterine lining in ways that make separation more likely, though the exact mechanism isn’t fully worked out.
Retroplacental Hematoma: A Different Type
Not all pregnancy hematomas are subchorionic. A retroplacental hematoma forms when blood collects directly behind the placenta, between the placenta and the uterine wall. This is essentially what happens during a placental abruption, where the placenta begins to peel away from its attachment site.
Known triggers for retroplacental bleeding include uterine fibroids located behind the placenta and, rarely, needle puncture during procedures like amniocentesis. When blood accumulates in this location, the growing hematoma can compress the surrounding tissue and compromise blood flow to the baby. In some cases, the separation happens slowly over weeks or months, a pattern called chronic abruption, where the bleeding stays hidden behind the placenta without visible vaginal bleeding.
How Size Affects the Outlook
Hematoma size relative to the gestational sac is the single most important factor in predicting how things will go. Providers classify them into three categories based on the percentage of the gestational sac they occupy:
- Small (under 20% of the sac): The most common and the most reassuring.
- Medium (20% to 50%): Worth monitoring closely but still generally favorable.
- Large (over 50%): Significantly higher risk of complications. Hematomas larger than 50 mL in volume also worsen the prognosis regardless of percentage.
The numbers are striking. In one study, pregnancies with hematomas smaller than 50% of the sac had only a 7% loss rate. When the hematoma reached 50% or more, two-thirds of those pregnancies ended in miscarriage. That means the vast majority of women with small to medium hematomas go on to carry to term, but large ones require close surveillance.
What It Means for the Rest of Pregnancy
A common worry is whether an early hematoma sets you up for problems later, like preterm birth or preeclampsia. A systematic review and meta-analysis published in the American Journal of Obstetrics and Gynecology looked at this directly and found reassuring results: subchorionic hematomas discovered in the first trimester were not significantly associated with preterm delivery, fetal growth restriction, cesarean delivery, or preeclampsia in singleton pregnancies. The odds ratio for preterm delivery was 1.11, which is statistically indistinguishable from no increased risk at all.
This is important context. While the hematoma itself can be alarming, especially if it causes visible bleeding, the evidence suggests that for single pregnancies, an early small-to-moderate hematoma does not meaningfully raise the risk of complications down the road.
What Happens After Diagnosis
There is no medication or procedure that resolves a subchorionic hematoma. The body either reabsorbs the blood over time or it passes as vaginal bleeding. Most hematomas resolve on their own within a few weeks.
Some providers recommend reduced physical activity or pelvic rest, though there’s no strong clinical evidence that bed rest changes outcomes. What typically happens is a follow-up ultrasound in one to two weeks to check whether the hematoma is stable, shrinking, or growing. If it’s getting smaller, that’s a good sign. If it’s growing or if bleeding increases, more frequent monitoring may be warranted.
The color of any vaginal bleeding can give you a rough sense of timing. Dark brown or black blood usually means the hematoma is old and being reabsorbed. Bright red blood suggests active bleeding. Both can be normal with a known hematoma, but a sudden increase in volume or the onset of cramping alongside heavy bleeding is worth immediate medical attention.