Repeated miscarriages at 6 weeks are most often caused by chromosomal problems in the embryo, but when it happens more than once, there may be an underlying factor in your body that keeps tipping the odds against a healthy pregnancy. Around 50 to 60% of all first-trimester miscarriages involve chromosomal abnormalities in the embryo, and that number holds true even in the 5-to-7 week window. The remaining cases point to a range of identifiable causes, many of which are treatable.
If you’ve had two or more consecutive losses, you meet the U.S. clinical definition of recurrent pregnancy loss. In the UK, the threshold is three. Either way, reaching that number typically opens the door to a full workup to look for treatable causes.
Why Chromosomal Problems Dominate Early Losses
At 6 weeks, an embryo is undergoing rapid cell division, and any major error in its chromosome count usually stops development cold. The most common abnormality is an extra copy of a single chromosome (called a trisomy), which accounts for roughly 57% of chromosomally abnormal miscarriages. Other errors include missing or extra sex chromosomes and duplications across multiple chromosomes.
These errors are largely random. They happen during egg or sperm formation and become more frequent as egg quality declines with age. A 20-year-old woman has a relatively low chance of a chromosomal mismatch in any given pregnancy, while a 40-year-old faces a substantially higher one. When miscarriage tissue is tested after a loss, finding a chromosomal cause can actually be reassuring: it suggests the loss was a one-off genetic accident rather than a sign of a chronic problem in your body.
Low Progesterone and the Luteal Phase
For the first several weeks of pregnancy, your body relies on a temporary structure called the corpus luteum to produce progesterone, the hormone that maintains the uterine lining and supports the embryo. The placenta doesn’t take over progesterone production until later. If the corpus luteum fails or doesn’t produce enough progesterone, the pregnancy can’t sustain itself.
A condition called luteal phase deficiency describes cycles where this progesterone-producing phase is abnormally short, typically under 10 days. About 8 to 9% of normally menstruating women have cycles that meet this definition. Signs include a short gap between ovulation and your period, or spotting in the days before your period starts. That said, reproductive medicine experts note that luteal phase deficiency has not been conclusively proven as an independent cause of recurrent loss, so it remains somewhat controversial as a standalone diagnosis. Your doctor may still check progesterone levels during the luteal phase to rule it out.
Thyroid Problems
Your thyroid gland plays a direct role in early pregnancy, and even mildly abnormal function can raise miscarriage risk. Women who have thyroid antibodies (a marker of autoimmune thyroid disease) appear especially vulnerable. For these women, research suggests that a TSH level above 2.5 can be associated with higher miscarriage and preterm delivery rates, while treatment with thyroid hormone may help. For women without thyroid antibodies, the threshold where treatment shows clear benefit is higher, around 4.0.
Thyroid screening is a standard part of the recurrent miscarriage workup and one of the more straightforward problems to address.
Antiphospholipid Syndrome
Antiphospholipid syndrome (APS) is an autoimmune condition where your immune system produces antibodies that promote abnormal blood clotting. In pregnancy, this can interfere with blood flow to the developing placenta, starving the embryo. APS is one of the few treatable causes of recurrent miscarriage with strong evidence behind it.
Diagnosis requires both a clinical event (such as three or more consecutive losses before 10 weeks) and positive blood tests for specific antibodies: lupus anticoagulant, anticardiolipin antibodies, or anti-beta-2 glycoprotein I antibodies. These antibodies must be found at moderate-to-high levels on two separate occasions at least 12 weeks apart, because temporary positive results can occur during infections or stress. Low-level or one-time positive results do not meet the diagnostic threshold. Once confirmed, APS in pregnancy is typically managed with blood-thinning medication, which significantly improves outcomes.
Uterine Shape Abnormalities
The shape of your uterus can affect whether an embryo implants in a well-nourished spot and whether the pregnancy has room to grow. A 2024 meta-analysis found that congenital uterine anomalies raise first-trimester miscarriage risk by about 62% overall, but some shapes carry far more risk than others.
A uterus with a partial wall of tissue dividing the cavity (called a subseptate uterus) had the highest odds of miscarriage, with roughly six times the risk compared to a normally shaped uterus. A fully septate uterus carried about three times the risk, a T-shaped uterus about five times, and a bicornuate (heart-shaped) uterus about twice the risk. The common thread is reduced cavity space and abnormal blood supply in the area where the embryo tries to implant. These anomalies are usually diagnosed with imaging, and some can be corrected surgically.
Inherited Clotting Disorders
Beyond APS, some women carry inherited gene variants that make their blood clot more easily than normal. The best known is Factor V Leiden, a mutation that makes a clotting protein resistant to being switched off by the body’s natural anticoagulant system. The result is a tendency toward excessive clotting, which can lead to tiny blood clots in the developing placenta and cut off the embryo’s blood supply.
A meta-analysis of nearly 3,000 patients found that high levels of sperm DNA damage doubled the risk of miscarriage, suggesting that the male partner’s contribution can also play a role, though testing for this is not yet part of standard workups.
Current guidelines do not recommend routine screening for inherited clotting disorders in women with recurrent loss alone, because the correlation is inconsistent. Testing is more likely to be ordered if you have a personal or family history of blood clots.
What Testing Looks Like
A recurrent miscarriage workup typically includes several categories of investigation. Blood tests screen for antiphospholipid antibodies, thyroid function, and sometimes hormonal levels like progesterone. If tissue from a miscarriage is available, chromosomal analysis of that tissue can reveal whether the loss was caused by a genetic error in the embryo, and whether parental chromosome testing (called a karyotype) is warranted.
Imaging of your uterus, usually with ultrasound or a specialized scan that uses saline to outline the cavity, checks for structural abnormalities like a septum or fibroids. Some clinics also test for blood sugar regulation and other metabolic factors. Testing for inherited thrombophilia is reserved for women with additional risk factors. Immune panels looking at natural killer cells or cytokines are sometimes offered, but evidence for their usefulness remains insufficient.
Timing matters for some of these tests. Clotting-related blood work, for example, is best done at least six weeks after a pregnancy loss to avoid results skewed by the pregnancy itself.
The Odds of a Successful Pregnancy
Even after multiple losses, the statistics are more hopeful than many people expect. On average, women with unexplained recurrent miscarriage have a 75% live birth rate in their next pregnancy. The miscarriage risk is concentrated in the first 9 weeks (about 20%), dropping to around 5% after that point.
Age and number of prior losses both influence these numbers. A 30-year-old woman with two previous unexplained miscarriages has roughly an 84% chance of a successful next pregnancy. With five prior losses, that drops to about 71%, still a majority. A 20-year-old after three losses has about a 90% chance of success, while a 45-year-old in the same situation has closer to 54%.
When a specific cause is identified and treated, such as APS or a uterine septum, the odds improve further. Even in cases where no cause is found (which happens in roughly 50% of couples), supportive care and close monitoring in early pregnancy are associated with better outcomes than no intervention at all.