A subchorionic hemorrhage happens when blood collects between the chorion (the outermost membrane surrounding the embryo) and the wall of the uterus. It affects roughly 0.5% to 22% of pregnancies depending on the population studied, with one large study of fertility patients finding a prevalence of 12.5%. The core cause is a partial separation of the chorionic membrane from the uterine lining, but several biological factors can set that separation in motion.
How the Bleeding Starts
During early pregnancy, the outer layer of the developing placenta sends specialized cells into the uterine lining to establish a blood supply. When that invasion process is impaired, or when new blood vessels in the uterine lining don’t form properly, the vessels that do exist tend to be more fragile. Fragile vessels are more likely to rupture as the placenta grows and embeds itself deeper into the uterine wall.
Once bleeding begins, blood follows the path of least resistance and pools in the space between the chorion and the uterine lining (called the decidua). The body tries to stop the bleed through normal clotting mechanisms: blood vessels constrict, platelets clump together, and fibrin (a protein involved in clot formation) accumulates. But in some cases, small veins in that subchorionic space become blocked, trapping blood rather than allowing it to drain. Blood flow can also form swirling patterns that encourage more fibrin buildup, which further prevents drainage and allows the pocket of blood to grow into a visible hematoma on ultrasound.
Why Some Pregnancies Are More Vulnerable
No single cause explains every case. Many subchorionic hemorrhages are discovered incidentally on a routine first-trimester ultrasound in women with no obvious risk factors. That said, researchers have identified several conditions that appear to raise the likelihood.
IVF and Assisted Reproduction
Pregnancies conceived through IVF show noticeably higher detection rates. One 2014 study found subchorionic hematomas in 22.4% of IVF pregnancies compared to 11% of naturally conceived pregnancies. Part of this gap may simply be surveillance bias: IVF patients get ultrasounds earlier and more often, so small hematomas that would go unnoticed in a typical pregnancy are caught on the screen. But some researchers believe IVF pregnancies carry a higher rate of abnormal placentation, meaning the placenta’s initial attachment to the uterine wall may be slightly different, making blood vessels more prone to rupture during that early embedding phase. This remains debated, with some animal and human studies showing placentation looks similar regardless of how pregnancy was conceived.
Abnormal Placental Development
The strongest biological explanation centers on what happens at the cellular level during placentation. Cells called syncytiotrophoblasts, which form the outer layer of the developing placenta, sometimes invade the uterine lining too aggressively or in a disorganized way. When this is paired with poor blood vessel formation in the surrounding tissue, the result is separation between the placental membranes and the uterine wall, bleeding into that gap, and eventually the peeling away of fetal membranes from the uterine surface.
Maternal Age and Other Factors
Advanced maternal age is sometimes listed as a risk factor, but the evidence is mixed. At least one study comparing women with and without subchorionic hematomas found no statistically significant difference in age between the two groups. Other commonly mentioned associations include a history of recurrent miscarriage, high blood pressure, clotting disorders, and uterine abnormalities, though large-scale data confirming each of these as independent causes is limited.
What a Subchorionic Hemorrhage Feels Like
Some women notice vaginal bleeding or spotting, which is often what prompts the ultrasound that reveals the hematoma. The bleeding can range from light brown discharge to bright red, and it sometimes comes with mild cramping. But many subchorionic hemorrhages produce no symptoms at all and are found only because an ultrasound was done for another reason. The absence of bleeding doesn’t mean the hematoma is smaller or less significant; it simply means the blood hasn’t found a path out through the cervix.
How Size Affects Outlook
Smaller hematomas generally carry a better prognosis. Many shrink on their own over a few weeks as the body reabsorbs the trapped blood. There is no set timeline for resolution, and follow-up ultrasounds are typically used to track whether the hematoma is growing, stable, or shrinking. Larger hematomas, particularly those that occupy a significant portion of the gestational sac, are associated with a higher risk of complications like miscarriage, preterm delivery, and placental problems later in pregnancy.
In one clinical study, 93 out of 100 women with ultrasound-confirmed subchorionic hematomas carried their pregnancies successfully, putting the miscarriage rate at 7% in that group. This is reassuring, though the rate varies depending on the size of the hematoma and how early in pregnancy it appears.
Treatment Options
There is no procedure that drains or removes a subchorionic hematoma. Management is largely expectant, meaning the approach is to monitor and wait. Some providers recommend reduced physical activity or pelvic rest, though evidence that this speeds resolution is limited.
Progesterone supplementation is one active treatment that has shown promise. In a study of 100 women with subchorionic hematomas treated with oral progesterone, the miscarriage rate was 7%, compared to 18.7% in a previous study of women treated with a different form of progesterone. The theory is that progesterone helps maintain the immune environment needed for pregnancy to continue, shifting the body’s immune response toward one that supports the developing embryo rather than attacking the foreign tissue. This is not yet standard care everywhere, and practices vary by provider.
Most subchorionic hematomas diagnosed in the first trimester resolve without lasting harm. The key variables that influence outcome are the hematoma’s size relative to the gestational sac, whether it continues to grow on serial ultrasounds, and how far along the pregnancy is when it’s first detected.