Passing blood clots during early pregnancy is one of the most alarming experiences expectant parents face, and it is a hallmark feature of what doctors call a threatened miscarriage. A threatened miscarriage means vaginal bleeding has occurred before 20 weeks of gestation while the cervix remains closed and the pregnancy is still viable. Research has identified the passage of clots as one of several signs that raise concern about whether the pregnancy will continue, though the majority of threatened miscarriages do ultimately result in a live birth. What the clots look like, how much bleeding accompanies them, and what an ultrasound reveals all shape the picture.
What “Threatened Miscarriage” Actually Means
The term sounds more dire than it often turns out to be. A threatened miscarriage is a clinical label applied any time vaginal bleeding occurs in the first half of pregnancy while the cervix is still closed and a fetal heartbeat can be detected. It is distinct from an inevitable miscarriage, where the cervix has begun to open, and from a missed miscarriage, where the embryo has stopped developing but has not yet been expelled. Threatened miscarriage affects a sizable share of confirmed pregnancies, and while the bleeding itself is distressing, the diagnosis alone does not mean loss is certain or even likely.
The bleeding can range from light spotting to flow resembling a normal period. Some women pass small clots; others experience only pink or brown discharge. A study examining the predictive value of symptoms at presentation found that passing clots vaginally, bleeding similar to a normal menstrual period, increasing bleeding over time, maternal age over 35, and a mismatch of four or more weeks between uterine size and expected gestational age were all significant predictors of a nonviable outcome.
Why Clots Form in the First Place
Blood clots during a threatened miscarriage often originate from a subchorionic hematoma, a pocket of blood that collects between the outer membrane surrounding the embryo and the uterine wall. This happens when the chorion partially separates from the underlying tissue, allowing blood to pool in the gap.
A subchorionic hematoma does not always cause noticeable bleeding. Some are discovered incidentally on a routine ultrasound, while others announce themselves with bright red blood or dark clots. The clots you see are essentially that pooled blood making its way out. Think of it as a bruise between the pregnancy sac and the uterus: the blood has to go somewhere, and when it drains vaginally, it can come out as clots, sometimes mixed with fresh red blood.
Not every episode of clot-passing traces back to a subchorionic hematoma. Bleeding can also come from the cervix itself, which becomes engorged with blood vessels during pregnancy and can bleed after intercourse or a pelvic exam. Cervical polyps, vaginal infections, and, less commonly, an ectopic pregnancy can all produce bleeding that mimics a threatened miscarriage. Distinguishing between these possibilities is one reason early ultrasound is so important: it confirms the pregnancy is inside the uterus, checks for a heartbeat, and can reveal whether a hematoma is present.
Does a Subchorionic Hematoma Make Miscarriage More Likely?
The short answer is yes, modestly. In a study of women presenting with first-trimester bleeding, about 30% of those with an ultrasound-confirmed subchorionic hematoma went on to miscarry, compared with roughly 13% of those without one. That difference is real, but it also means that roughly seven out of ten pregnancies with a hematoma still continued.
What gets murkier is whether the size of the hematoma matters. Some research has found that larger hematomas are linked to higher miscarriage rates. One prospective cohort study reported that as hematoma size increased, the chance of miscarriage rose significantly, and that an anterior placenta appeared to be a protective factor. But other studies have found no clear relationship between hematoma size and pregnancy outcome. One study, for instance, found no statistically significant link between size, location, or gestational age at diagnosis and whether the pregnancy survived. Another found that in about four out of five women with hematomas, the blood collection measured under five centimeters, and no meaningful correlation between size and adverse outcomes emerged.
The conflicting evidence likely reflects differences in how hematomas were measured, when in pregnancy women were scanned, and how large the study populations were. The honest takeaway is that a subchorionic hematoma raises the risk somewhat, but doctors cannot reliably use its size on a single scan to predict exactly what will happen next.
When Clots Are a Worse Sign
Certain patterns of bleeding and clot-passing carry more concern than others. A one-time episode of light spotting or a small clot is different from escalating bleeding that soaks through pads and involves large, tissue-like clots. The study on predictive signs found that bleeding that was increasing over time was a more worrying indicator than a single bleed that tapered off. Similarly, when the uterus measured substantially smaller than expected for the gestational age, that discrepancy signaled a higher chance that the pregnancy was no longer developing normally.
Context matters too. A woman over 35 who passes clots is statistically at higher risk than a younger woman with the same symptom, partly because chromosomal abnormalities in the embryo become more common with maternal age. And while a subchorionic hematoma accounts for many cases of clot-related bleeding, there are scenarios where clots represent the beginning of an actual miscarriage in progress rather than a contained bleed that can resolve. If cramping intensifies, the cervix begins to dilate, or tissue is passed along with the clots, the situation has typically moved beyond “threatened” into a different clinical category.
How Doctors Evaluate What Is Happening
The first step is almost always a transvaginal ultrasound. This confirms whether the pregnancy is in the uterus, whether a heartbeat is present, and whether a subchorionic hematoma or other structural issue is visible. Real-time ultrasound can also help distinguish a viable early pregnancy from an ectopic pregnancy with a decidual cast, a thickened uterine lining that can mimic the appearance of a gestational sac on lower-quality imaging.
Blood tests typically complement the ultrasound. Measuring both progesterone and beta-hCG (the “pregnancy hormone”) together gives a more reliable picture than either test alone. Research has found that combining the two markers yields a diagnostic accuracy of about 86% in predicting whether a threatened miscarriage will progress to an actual loss, outperforming either hormone measured on its own. Low and falling levels of both hormones together are a more ominous sign than low levels of just one.
In practice, doctors often order serial blood draws 48 hours apart to see whether hCG levels are rising appropriately. A healthy early pregnancy typically sees hCG roughly double every two to three days. When levels plateau or drop, the prognosis worsens. Combined with what the ultrasound shows, this hormone trend helps guide whether the approach is watchful waiting or preparation for a likely loss.
Treatment Options and What the Evidence Supports
For decades, the standard advice for threatened miscarriage was bed rest. That advice persists in many settings despite a lack of evidence behind it. A Cochrane review found insufficient high-quality evidence that bed rest prevents miscarriage in women with confirmed fetal viability and vaginal bleeding in the first half of pregnancy. Separate research has underscored this point, finding no difference in miscarriage rates between bed rest and no bed rest, and even flagging a psychological risk: women who are told to rest and then miscarry may blame themselves for not resting enough, when the evidence suggests it would not have changed the outcome.
Progesterone supplementation is the intervention with the most active research behind it. A Cochrane review of seven trials found that progestogen treatment probably reduces the risk of miscarriage compared to placebo, with a risk ratio of about 0.64. However, the picture is more nuanced than that single number suggests. A large analysis of randomized evidence found that the benefit of vaginal micronized progesterone was concentrated among women who had both current bleeding and a history of prior miscarriage. For women with three or more previous losses who were also bleeding in the current pregnancy, the live birth rate was about 72% with progesterone versus 57% with placebo. For women without a history of recurrent loss, the benefit was much smaller or absent. A separate placebo-controlled trial, the STOP trial, found no significant difference in live birth rates between progesterone and placebo in women with threatened miscarriage overall, including among those with one or two prior losses.
What this means practically is that progesterone is not a blanket fix for threatened miscarriage, but it does appear to help a specific group: women who are bleeding in early pregnancy and have experienced previous losses. If that describes your situation, vaginal progesterone is a conversation worth having with your provider.
Tranexamic Acid and Other Emerging Approaches
Tranexamic acid (TXA), a medication that helps blood clot and is widely used for heavy menstrual bleeding and postpartum hemorrhage, has attracted interest as a potential treatment for first-trimester bleeding. A scoping review found that the evidence base for TXA in early pregnancy is still thin: only four small studies totaling 319 patients met criteria for evaluating whether it reduces bleeding or improves pregnancy outcomes. On the safety side, data from over 76,000 patients across 30 studies showed no significant increase in blood clots with TXA use, and among 234 pregnancies exposed to TXA outside of the postpartum setting, 98% resulted in live births. But most of that safety data comes from studies focused on bleeding after delivery rather than first-trimester use specifically. Larger, well-designed trials are needed before TXA becomes a routine recommendation for threatened miscarriage.
Some women turn to complementary approaches. Chinese herbal medicines, acupuncture, and nutritional supplements have all been used for threatened miscarriage, and some case series report favorable outcomes. But safety concerns persist because many herbal preparations have not been rigorously tested in pregnancy, and the quality of existing evidence is generally low. Any complementary treatment should be discussed with a provider who can weigh potential interactions and risks.
What Happens to the Rest of the Pregnancy
Even when a threatened miscarriage resolves and the pregnancy continues, the episode is not always without consequences later on. Research comparing women who experienced threatened miscarriage with matched controls found significantly higher rates of preterm delivery, low birth weight, and premature rupture of membranes in the threatened miscarriage group. The miscarriage rate itself was about 17% in the study group versus roughly 2% in controls, and preterm delivery occurred at about 16% versus 2%.
These numbers do not mean that every pregnancy following a threatened miscarriage will have complications. The majority still reach term. But they do suggest that your care team may monitor you more closely in the second and third trimesters if you experienced significant bleeding early on, particularly if a subchorionic hematoma was identified. Growth scans, cervical length checks, and attention to signs of preterm labor are all reasonable precautions in this context.
The Thrombophilia Connection
An underappreciated aspect of subchorionic hematomas is their potential link to inherited clotting disorders. A case series described three women who developed subchorionic hematomas and were subsequently found to have thrombophilias: one had a genetic mutation affecting folate metabolism, and the other two had protein S deficiency. All three pregnancies ended in loss. The authors proposed that the hematoma itself may sometimes be the first visible sign of an abnormal clotting state in the mother, and suggested that women with subchorionic hematomas, especially recurrent ones, may benefit from a thrombophilia workup.
This is a small case series, not a definitive study, and most subchorionic hematomas occur in women without any identifiable clotting disorder. But for women who have experienced recurrent pregnancy loss with subchorionic hematomas, testing for conditions like antiphospholipid syndrome, factor V Leiden, or protein S and C deficiencies is a reasonable step. If a thrombophilia is identified, treatment with low-dose aspirin or anticoagulants during future pregnancies may be an option, though that decision involves a specialist.
Rh-Negative Blood Type and Anti-D
If you have Rh-negative blood, any episode of vaginal bleeding in pregnancy, including a threatened miscarriage, raises the question of whether you need an anti-D immunoglobulin injection. The concern is that if the baby has Rh-positive blood and some of it enters your circulation during bleeding, your immune system could develop antibodies that attack fetal red blood cells in this or a future pregnancy. Clinical guidance recommends that anti-D immunoglobulin be offered to all non-immune Rh-negative women who present with threatened miscarriage, even before 12 weeks of gestation.
This is a straightforward, low-risk intervention that prevents a potentially serious problem in future pregnancies. If you know you are Rh-negative and experience any bleeding, mention it to your care team even if you assume they already know your blood type.
The Emotional Weight of Waiting
One of the hardest parts of a threatened miscarriage is the uncertainty. You are bleeding, possibly passing clots, and no one can guarantee the outcome. The psychological toll is real and well documented. Research found that about a third of women facing threatened miscarriage met criteria for major depressive symptoms, compared with 17% of women with uncomplicated pregnancies. Nearly half had significant anxiety symptoms, versus about a quarter of controls. Even after adjusting for other risk factors, threatened miscarriage roughly tripled the odds of depressive symptoms in the women experiencing it.
A separate study found that stress, anxiety, and intolerance of uncertainty were collectively responsible for over half the variation in psychological well-being among women with threatened miscarriage. Partners, interestingly, did not show the same elevated rates of depression in the research, though that does not mean they are unaffected.
If you are going through this, the anxiety is not a character flaw or an overreaction. It is a predictable response to a genuinely uncertain and frightening situation. Seeking mental health support during this time is not premature, and some providers now screen for anxiety and depression as part of threatened miscarriage follow-up.
Racial Disparities in Early Pregnancy Outcomes
The experience of threatened miscarriage does not fall equally across all groups. A narrative review of racial disparities in early pregnancy found that Black women face a higher incidence of miscarriage and ectopic pregnancy, along with poorer survival rates from gestational trophoblastic disease. These disparities likely reflect a combination of systemic factors: differences in access to timely care, chronic stress from structural racism, higher rates of conditions like fibroids that can complicate early pregnancy, and documented gaps in how seriously symptoms are taken in clinical settings.
For anyone navigating threatened miscarriage, advocating for yourself matters. If your bleeding is worsening, if you feel your concerns are being minimized, or if you are not getting clear answers about what the ultrasound and blood work show, pushing for a second opinion or a different provider is entirely reasonable. Early pregnancy units, where they exist, tend to offer more specialized and timely evaluation than general emergency departments for this exact situation.