Recurrent miscarriage affects 1 to 2 percent of women of childbearing age, and in roughly half of those cases, no clear cause is ever identified. That statistic can feel maddening when you’re living it, but it also means that in the other half, there is a treatable or identifiable reason. Understanding the full range of causes can help you have more focused conversations with your doctor and pursue the right testing sooner.
Even after three or more losses, the odds are still in your favor. One study tracking women with unexplained recurrent miscarriage found that nearly 70% eventually had a live birth. That number is worth holding onto while you work through the possible explanations below.
How Recurrent Miscarriage Is Defined
Most specialists define recurrent miscarriage as three or more consecutive first-trimester losses with the same biological father. Some clinicians begin investigating after two losses, especially if you’re over 35 or if the losses occurred later in pregnancy. The workup typically includes blood tests, imaging of the uterus, and sometimes genetic testing of both partners.
Chromosomal Problems in One or Both Parents
The most common reason for any single miscarriage is a random chromosomal error in the embryo. These one-off errors become more frequent with age, but they aren’t usually the explanation for repeated losses. What can cause a pattern is something called a balanced translocation, where a parent carries rearranged chromosomes that function fine in their own body but produce unbalanced combinations when passed to an embryo. Balanced translocations are found in roughly 1 to 5 percent of couples with recurrent loss. A simple blood test called a karyotype, done on both partners, can detect this. If a translocation is found, IVF with genetic screening of embryos before transfer is one option that significantly improves the chance of a healthy pregnancy.
Uterine Shape and Structure
The physical shape of your uterus plays a larger role than many people realize. Anatomical uterine anomalies show up in 15 to 42 percent of women with recurrent loss, depending on the study and how thoroughly imaging is done. These fall into two categories: conditions you’re born with and conditions that develop over time.
The most common congenital issue is a septate uterus, where a wall of tissue partially divides the uterine cavity. It’s found in 6 to 16 percent of recurrent miscarriage cases. The septum has poor blood supply, so an embryo that implants on it may not get the nutrients it needs to survive. Surgical removal of the septum is a relatively straightforward procedure that can improve outcomes.
Acquired problems like fibroids, polyps, or scar tissue from previous surgeries account for another 6 to 15 percent of cases. Not all fibroids cause miscarriage. Location matters more than size: fibroids that distort the inside of the uterine cavity are the ones most likely to interfere with implantation and pregnancy. An imaging study such as a saline ultrasound or an MRI can clarify whether a structural problem is contributing to your losses.
Antiphospholipid Syndrome
Antiphospholipid syndrome (APS) is an autoimmune condition where your immune system produces antibodies that increase blood clotting. During pregnancy, these tiny clots can form in the placenta and cut off blood flow to the developing baby. APS is one of the most important treatable causes of recurrent miscarriage.
Diagnosis requires both a clinical history of pregnancy loss and positive blood tests for specific antibodies on two occasions at least 12 weeks apart. The key tests look for lupus anticoagulant, anticardiolipin antibodies, and anti-beta-2 glycoprotein I antibodies. A single positive result isn’t enough for diagnosis because these antibodies can appear temporarily during infections or stress. Persistence is what defines the syndrome. Once confirmed, treatment with low-dose aspirin and a blood-thinning injection during pregnancy substantially improves the live birth rate.
Thyroid and Hormonal Imbalances
Your thyroid sets the metabolic pace for nearly every system in your body, including reproduction. Overt hypothyroidism (a clearly underactive thyroid) increases the risk of miscarriage, infertility, and complications later in pregnancy. Even subclinical hypothyroidism, where your thyroid is only mildly underperforming, has been linked to pregnancy loss in older research. A simple blood test measuring thyroid-stimulating hormone (TSH) and thyroid antibodies is part of the standard recurrent miscarriage workup.
Other hormonal factors matter too. Poorly controlled diabetes and polycystic ovary syndrome (PCOS) can both interfere with early pregnancy. Progesterone, the hormone responsible for maintaining the uterine lining in early pregnancy, has received significant attention. A large clinical trial found that women with a history of three or more miscarriages who also had bleeding in their current early pregnancy benefited most from vaginal progesterone supplementation. The treatment was started before six weeks of gestation and continued through 12 weeks. For women with fewer prior losses or no current bleeding, the benefit was less clear.
Chronic Infection of the Uterine Lining
Chronic endometritis is a low-grade, ongoing infection of the uterine lining that often causes no obvious symptoms. You might have no pain, no fever, and no unusual discharge, yet the persistent inflammation can prevent an embryo from implanting properly or surviving the early weeks. Studies have found chronic endometritis in about 27 percent of women with recurrent pregnancy loss.
The condition is diagnosed through a biopsy of the uterine lining, where a pathologist looks for specific immune cells that signal chronic inflammation. The good news is that treatment is straightforward: a two-week course of antibiotics. Research shows a dramatically improved live birth rate in women whose chronic endometritis was identified and treated compared to women whose infection went undetected or untreated. Despite these promising results, testing for chronic endometritis is not yet a routine part of every recurrent miscarriage workup, so it’s worth asking about.
Sperm DNA Damage
Most recurrent miscarriage evaluations focus entirely on the woman, but the male partner contributes half the embryo’s genetic material. A meta-analysis of 13 studies found that male partners of women with recurrent pregnancy loss had significantly higher rates of sperm DNA fragmentation compared to partners of women without losses. Sperm with fragmented DNA can still fertilize an egg, but the resulting embryo may develop abnormally once the father’s genes activate, typically around day three after fertilization.
The egg has some ability to repair minor DNA damage in sperm, but when the damage is extensive, it overwhelms that repair capacity and can lead to failed implantation or miscarriage. Sperm DNA fragmentation testing is not yet standard, but it’s increasingly available. Factors that worsen fragmentation include smoking, heat exposure, obesity, infections, and long intervals between ejaculations. In some cases, lifestyle changes alone can improve sperm DNA quality enough to make a difference.
Blood Clotting Disorders
Inherited clotting disorders (thrombophilias) like Factor V Leiden and the prothrombin gene mutation have a complicated relationship with pregnancy loss. A large meta-analysis of 89 studies involving over 30,000 women found an association between these mutations and miscarriage in both the first and second trimesters. Protein S deficiency was specifically linked to late-term fetal loss. However, the absolute risk increase is small, and major medical organizations, including the American College of Obstetricians and Gynecologists, currently recommend against routine screening for inherited thrombophilias in women with recurrent miscarriage.
The reason for this cautious stance is that studies haven’t clearly shown that blood-thinning treatment improves live birth rates in women who carry these mutations. This is distinct from antiphospholipid syndrome, which is an acquired (not inherited) clotting problem and does have proven treatment. If you have a strong family history of blood clots or have experienced late pregnancy losses, your doctor may still consider testing on a case-by-case basis.
When No Cause Is Found
After a full evaluation, about half of couples with recurrent miscarriage receive no definitive diagnosis. This is called unexplained recurrent pregnancy loss, and while the label is frustrating, the prognosis is actually encouraging. The study that tracked these couples found that roughly 70% achieved a live birth over time, though the probability was somewhat lower for women who had experienced three or more losses compared to those with two. Supportive care in a dedicated early pregnancy clinic, including regular ultrasounds and emotional support in the first trimester, has itself been associated with better outcomes in unexplained cases. The simple reassurance of being closely monitored appears to reduce stress hormones that may play a role in early pregnancy survival.