There is no single universally agreed-upon “average” size for a subchorionic hematoma because these blood collections vary enormously from one pregnancy to another, and researchers measure them in different ways. One commonly referenced classification divides them into small (6–14 mm), moderate (15–27 mm), and large (28–58 mm) based on caliper measurements taken across two ultrasound planes.1European Journal of Obstetrics & Gynecology and Reproductive Biology. The significance of ultrasound features of sub-chorionic haemorrhage as a predictor of adverse perinatal outcome: A retrospective review But absolute millimeter measurements tell only part of the story, because the same 20 mm hematoma looks very different next to a tiny six-week gestational sac than it does next to a twelve-week sac. That is why most clinicians care less about the raw number and more about the hematoma’s size relative to the pregnancy.
What a Subchorionic Hematoma Actually Is
A subchorionic hematoma forms when blood collects between the chorion, the outermost membrane surrounding the embryo, and the decidua, the lining of the uterus. In plain terms, a small pocket of bleeding opens up where the placenta is attaching to the uterine wall.2PubMed Central. Subchorionic hematoma: Research status and pathogenesis On ultrasound it typically appears as a dark, crescent-shaped area tucked alongside the gestational sac. Some are barely visible slivers; others can grow large enough to rival the sac itself. Many are found incidentally during a routine early scan in someone who has had no bleeding at all, while others show up when a pregnant person presents with vaginal spotting or cramping.
Why There Is No Single “Average” in Millimeters
If you are searching for one neat number, the honest answer is that the research does not offer one, and for good reason. Subchorionic hematomas are irregularly shaped. They conform to the space between the membranes rather than forming tidy spheres, which means a single length measurement can be misleading. A hematoma that measures 25 mm in its longest axis might be paper-thin in another dimension, making it far smaller in actual volume than a rounder 20 mm collection. One study that tried to calculate hematoma volume from three perpendicular measurements found the results were unreliable precisely because the shapes were so irregular.3PubMed. Subchorionic Hematoma: Correlation of Grading Techniques With First-Trimester Pregnancy Outcome
To get around this problem, researchers have used a variety of grading systems over the years. Some measure three caliper dimensions. Some ask the sonographer to estimate whether the hematoma looks small, moderate, or large based on a visual impression. Others calculate the ratio of the hematoma’s largest diameter to the diameter of the gestational sac. Still others estimate what percentage of the sac’s circumference is bordered by blood.4Obstetrical & Gynecological Survey. Subchorionic Hematoma: Correlation of Grading Techniques With First-Trimester Pregnancy Outcome This patchwork of methods is one reason you will not find a clean average millimeter figure in any guideline. The field has not settled on a single measurement standard.
The Millimeter Ranges That Do Exist
The most concrete millimeter-based classification in recent literature comes from a retrospective study that sorted hematomas into three size bins based on caliper measurements: small at 6–14 mm, moderate at 15–27 mm, and large at 28–58 mm.1European Journal of Obstetrics & Gynecology and Reproductive Biology. The significance of ultrasound features of sub-chorionic haemorrhage as a predictor of adverse perinatal outcome: A retrospective review Case reports describe hematomas well beyond that upper range. One case series documenting persistent first-trimester hematomas measured them at dimensions like 23 × 17 mm, 31 × 27 mm, and 25 × 28 mm before treatment, all of which resolved over the following weeks.5PubMed Central. Alpha-Lipoic Acid and Leucine as Adjunct Therapy for Persistent First-Trimester Subchorionic Hematomas in Assisted Reproductive Technology Pregnancies Massive hematomas have been documented in case reports as well, sometimes extending far beyond these ranges and posing serious risks to the pregnancy.6PubMed Central. Successful Management of a Massive Second-Trimester Subchorionic Hematoma: A Case Report
So while “most” hematomas in clinical studies seem to fall roughly between 10 mm and 40 mm at their largest dimension, there is a long tail in both directions. Very small ones under 10 mm may not even be consistently reported because they are hard to distinguish from normal ultrasound shadowing, and very large ones push well past 50 mm.
Why Relative Size Matters More Than Absolute Millimeters
If your ultrasound report says the hematoma is 18 mm, the number alone does not tell your doctor much. What matters clinically is how large the hematoma is compared to the gestational sac or the developing pregnancy. A study that divided hematomas into small (ratio under one-quarter of the sac’s diameter), medium (one-quarter to one-half), and large (over one-half) found strikingly different outcomes across those groups.7Archives of Medical Science. How does subchorionic hematoma in the first trimester affect pregnancy outcomes? The large-ratio group had dramatically higher rates of preterm delivery, early pregnancy loss, and fetal growth restriction compared to the small and medium groups.
Another study using a percentage-based approach found a similar pattern. When the hematoma occupied a larger proportion of the gestational sac, miscarriage rates climbed.8PubMed Central. Association between the proportion of subchorionic hematoma within the gestational sac and miscarriage rate in IVF/ICSI patients The same direction emerged in a study comparing the fraction of the sac surrounded by the hematoma to first-trimester pregnancy loss: that fraction-based estimate was the grading method that correlated most strongly with outcome, outperforming both subjective impression and raw volume calculations.3PubMed. Subchorionic Hematoma: Correlation of Grading Techniques With First-Trimester Pregnancy Outcome
The practical takeaway: if your doctor or sonographer tells you the hematoma is “small relative to the sac,” that is more reassuring than any specific millimeter number. Conversely, a hematoma described as occupying more than half the sac’s diameter carries meaningfully higher risk even if its absolute measurement in millimeters sounds modest.
How Size Connects to Pregnancy Outcomes
Across multiple studies, the pattern is consistent: bigger hematomas correlate with worse outcomes, and the relationship appears dose-dependent. In a prospective cohort study, miscarriage rates rose significantly with increasing hematoma size. Medium and large hematomas were associated with earlier delivery, while small hematomas did not significantly differ from controls in delivery timing.9PubMed Central. Association between graded subchorionic hematoma and adverse pregnancy outcomes in singleton pregnancies: a prospective observational cohort study A separate study found that the miscarriage rate increased progressively with hematoma size in IVF patients.10Human Reproduction. Association between subchorionic hematoma’s size and miscarriage rate in IVF/ICSI patients
One detailed analysis broke things down further. In pregnancies with large hematomas (ratio over one-half of the sac), about 43% ended in early pregnancy loss, compared to roughly 13% for medium hematomas and 9% for small ones. Preterm delivery followed the same gradient, hitting about 43% in the large group but staying under 10% in the small group.7Archives of Medical Science. How does subchorionic hematoma in the first trimester affect pregnancy outcomes? Other complications like fetal growth restriction and placental abruption also climbed with hematoma size in that study.
A Japanese retrospective study tried to pin down useful cutoff values for “large.” Using the percentage of the gestational sac occupied by the hematoma, a cutoff of about 25% showed reasonable sensitivity and specificity for predicting complications. The group with larger hematomas had a preterm delivery rate of about 24%, compared to around 4% in the smaller-hematoma group.11SpringerOpen. Quantification of the size of subchorionic hematoma causing pregnancy-related complications: a retrospective cohort study
All of these studies have limits. They use different measurement methods and different definitions of “large,” which makes it hard to combine their numbers into one universal threshold. But the direction of evidence is clear: size matters, and relative size matters more than absolute size.
Timing and Location Are Also Important
Size is not the only factor that shapes outcomes. When the hematoma is diagnosed and where it sits within the uterus also play a role. A study of singleton pregnancies found that when a subchorionic hematoma was diagnosed before seven weeks of gestation, the adjusted odds of miscarriage before twenty weeks were roughly 2.7 times higher than when the diagnosis came later. When the hematoma reached its peak size before seven weeks, the odds were even higher, about 3.4 times that of later-peaking hematomas.12Scientific Reports. Association between first-trimester subchorionic hematoma and pregnancy loss before 20 weeks of gestation in singleton pregnancies
Location matters as well. A hematoma sitting behind the developing placenta, called a retroplacental hematoma, carries a higher miscarriage risk than one positioned elsewhere along the chorionic membrane.13PubMed Central. The Impact of Incidental Ultrasound Finding of Subchorionic and Retroplacental Hematoma in Early Pregnancy A retroplacental position was also linked to increased risk of adverse maternal and neonatal complications more broadly.14Obstetrics and Gynecology. Clinical significance of subchorionic and retroplacental hematomas detected in the first trimester of pregnancy So two hematomas of identical millimeter size can pose very different levels of risk depending on where exactly they formed.
Symptoms Do Not Always Match Size
You might assume a larger hematoma would always cause more bleeding, but the relationship between size and symptoms is not that straightforward. Many subchorionic hematomas, including moderate-sized ones, are discovered on ultrasound with no accompanying vaginal bleeding at all. On the flip side, even a small hematoma can cause alarming spotting if it is positioned near the cervical opening where blood can drain.
What does seem to matter is the combination of symptoms. In a study of infertile patients who conceived through assisted reproduction, having a subchorionic hematoma alone was not associated with first-trimester miscarriage. But among those with a hematoma, patients who experienced both bleeding and cramping had a substantially higher probability of miscarriage, around 62%, compared to about 12% in those without those symptoms.15PubMed Central. Outcomes of subchorionic hematoma-affected pregnancies in the infertile population The practical implication: if you have a known hematoma and develop both bleeding and cramping together, that combination deserves prompt medical attention regardless of the hematoma’s size on ultrasound.
How Hematomas Are Measured on Ultrasound and Why Precision Varies
If you have had serial ultrasounds, you may have noticed that the reported size of your hematoma shifts from scan to scan, sometimes by more than you would expect. Part of this is real change, but part of it is measurement variability. Standard two-dimensional ultrasound captures a flat cross-section of an irregularly shaped three-dimensional structure, and the numbers you get depend heavily on the angle of the probe and which plane the sonographer captures.
Newer three-dimensional techniques, such as virtual organ computer-aided analysis, can trace the entire outline of the hematoma and calculate its volume more accurately. One study found high agreement between different observers using this method, with correlation coefficients above 0.90.16PubMed. Subchorionic Hematoma Volume Measured by Virtual Organ Computer-Aided Analysis in the Prediction of First-Trimester Pregnancy Outcome Most clinics, though, still rely on standard two-dimensional measurements, which means the numbers on your report carry a margin of error. A hematoma reported at 22 mm one week and 19 mm the next may not have actually changed at all; it may just have been measured from a slightly different angle. This is worth keeping in mind if you are anxiously tracking millimeters between appointments.
Risk Factors for Developing a Subchorionic Hematoma
Subchorionic hematomas are more common in pregnancies conceived through assisted reproductive technology. A large retrospective study found that the incidence was about 28% in fresh IVF cycles and about 23% in frozen embryo transfer cycles, compared to around 13% in naturally conceived pregnancies.17PubMed Central. Incidence of subchorionic hematoma and contributing factors in assisted reproductive technologies—a retrospective cohort study Interestingly, that same study found that the presence of a hematoma was not significantly associated with miscarriage on its own.
Other identified risk factors include having polycystic ovary syndrome, a history of hydrosalpinx (fluid-filled fallopian tubes), and thin endometrial lining at the time of embryo transfer.18PubMed Central. Application of a Nomogram for Predicting the Risk of Subchorionic Hematoma in Early Pregnancy With In Vitro Fertilization-Embryo Transfer/Frozen Embryo Transfer Fresh embryo transfer also carried higher risk compared to frozen cycles. For those who conceived naturally, the usual suspects like advanced maternal age and prior pregnancy complications are generally discussed in clinical practice, though the evidence base is less robust than for IVF-specific risk factors.
How Most Hematomas Resolve
The majority of subchorionic hematomas, particularly small and moderate ones, resolve on their own without specific treatment. The body gradually reabsorbs the blood, and by the end of the first trimester or early second trimester, many are no longer visible on ultrasound. In the case series mentioned earlier, hematomas ranging from about 23 mm to 31 mm in their largest dimension resolved completely by 12 to 13 weeks of gestation.5PubMed Central. Alpha-Lipoic Acid and Leucine as Adjunct Therapy for Persistent First-Trimester Subchorionic Hematomas in Assisted Reproductive Technology Pregnancies While that series involved supplemental therapy, spontaneous resolution is well documented across the literature even without intervention.
What about treatment? The evidence is thin and somewhat contradictory. One older study of 230 patients found that those who adhered to bed rest had fewer spontaneous abortions (about 10% versus 23%) and a higher rate of term pregnancy compared to those who continued their usual activity.19PubMed. Pregnancy outcome of threatened abortion with subchorionic hematoma: possible benefit of bed-rest? But a randomized trial comparing bed rest alone to bed rest plus progesterone supplementation found no significant difference between the two groups after two weeks: about 46% of the bed-rest-only group and 42% of the progesterone group had resolution, and neither approach reduced the incidence of abortion.20Philippine Journal of Obstetrics and Gynecology. Dydrogesterone and bed rest vs. bed rest alone in the management of hemorrhage The bed rest study was not randomized, which limits how much weight you can give it. Most current practice involves activity restriction and watchful waiting, with follow-up ultrasounds to track whether the hematoma is growing or shrinking.
Twin Pregnancies and Subchorionic Hematomas
If you are carrying twins, the dynamics shift somewhat. A study of over 800 twin pregnancies found that about 10% had a subchorionic hematoma diagnosed before 14 weeks. Reassuringly, there were no significant differences in baseline characteristics or most pregnancy outcomes between those with and without a hematoma. However, having a hematoma was independently associated with a roughly threefold increase in the odds of needing a blood transfusion at delivery.21American Journal of Obstetrics and Gynecology. Subchorionic hematoma in twin pregnancies: prevalence, risk factors, and pregnancy outcomes Twin pregnancies already carry higher baseline risks for complications like preterm delivery, so a hematoma adds one more variable for your care team to monitor, even if it does not dramatically change the overall prognosis in the way a large hematoma might in a singleton pregnancy.
What to Ask Your Doctor
When you get an ultrasound report that mentions a subchorionic hematoma, the millimeter measurements can feel like the most important numbers in the world. But the raw size number is only one data point among several. Questions that will give you a clearer picture include: how large is the hematoma relative to the gestational sac? Is it positioned behind the placenta or elsewhere? Has it grown, stayed the same, or shrunk since the last scan? And are there any other findings on the ultrasound, like a slow fetal heart rate or an abnormal yolk sac, that might change the overall risk assessment?
It is also worth asking about the measurement method. If your report gives three dimensions, you are getting caliper measurements. If it gives a ratio or percentage, the sonographer was comparing the hematoma to the sac. Both approaches have value, but they are not directly comparable, so tracking changes over time works best when the same method is used at each visit. The anxiety that comes with a subchorionic hematoma diagnosis is real, and understanding what the numbers on your report mean, and what they do not mean, can help you have more productive conversations with your care team during what is often a stressful stretch of waiting.