About half of all early miscarriages are caused by chromosomal abnormalities in the embryo, meaning the pregnancy was never able to develop normally from the start. The other half involve a mix of factors including maternal age, chronic health conditions, immune disorders, infections, uterine structure, and certain lifestyle exposures. Most early miscarriages happen before 12 weeks, and for many, no single preventable cause can be identified.
Chromosomal Abnormalities
Roughly 50% of first-trimester miscarriages result from chromosomal errors in the embryo. These are random mistakes that happen when the egg and sperm combine or during the earliest cell divisions. The embryo ends up with too many or too few chromosomes, making normal development impossible.
Among miscarriages caused by chromosomal problems, about 86% involve numerical errors, meaning whole extra or missing chromosomes rather than smaller structural defects. The single most common type is autosomal trisomy, where the embryo has three copies of a chromosome instead of two. Trisomies of chromosomes 16 and 22 show up most frequently. These errors are not inherited and are not caused by anything either parent did. They occur more often as egg quality declines with age, which is one reason miscarriage rates climb after 35.
Maternal Age
Age is one of the strongest predictors of early pregnancy loss. A large 2019 study from Norway found that miscarriage risk is lowest between ages 25 and 29, then rises steadily after 30. By age 45, the risk of losing a pregnancy reaches about 53%. This steep increase is driven primarily by the rising rate of chromosomal errors in eggs as women get older. It does not mean that healthy pregnancies after 35 are rare, but it does mean the statistical odds of early loss increase with each passing year.
Chronic Health Conditions
Certain medical conditions raise the risk of miscarriage when they’re poorly controlled during the weeks around conception and early pregnancy. Uncontrolled diabetes is one of the most well-established: when blood sugar levels stay consistently high during the first trimester, the developing embryo is more vulnerable to growth disruption. Thyroid disorders, both overactive and underactive, also interfere with early pregnancy maintenance. Polycystic ovary syndrome (PCOS) and obesity are additional risk factors, likely because of their effects on hormonal balance and inflammation.
The key word in most of these cases is “uncontrolled.” When diabetes, thyroid disease, or other chronic conditions are well managed before and during pregnancy, the added risk drops substantially.
Immune System Factors
Antiphospholipid syndrome (APS) is one of the few treatable immune causes of recurrent miscarriage. In APS, the immune system produces antibodies that promote blood clotting in the tiny vessels of the placenta, cutting off the embryo’s supply of oxygen and nutrients. Three consecutive unexplained pregnancy losses before 10 weeks is one of the diagnostic criteria for this condition.
Research on preconception blood work has found that women with certain antiphospholipid antibodies had nearly five times the risk of very early embryonic loss compared to women without those antibodies. Women who tested positive for two or more types of these antibodies faced even higher risk. APS is significant because it’s identifiable through blood tests and, once diagnosed, can often be managed with blood-thinning treatment in subsequent pregnancies.
Uterine Structure
The physical shape of the uterus can affect whether a pregnancy survives the first trimester. A septate uterus, where a wall of tissue divides part of the uterine cavity, is the most common structural variation linked to miscarriage. The septum reduces the usable space inside the uterus and, more importantly, the tissue of the septum has poorer blood supply than normal uterine lining. An embryo that implants on or near the septum may not receive adequate nourishment. Most miscarriages in women with a septate uterus happen in the first trimester, suggesting the problem starts at implantation rather than later in pregnancy.
Submucosal fibroids, which are noncancerous growths that bulge into the uterine cavity, can also interfere with implantation or disrupt blood flow to the developing pregnancy. Not all fibroids cause problems. Location matters more than size.
Infections
Several infections can trigger miscarriage if contracted during early pregnancy. Three of the most significant are listeriosis, toxoplasmosis, and cytomegalovirus (CMV).
- Listeriosis comes from the bacterium found in ready-to-eat deli meats, hot dogs, meat pâté, and soft cheeses made from unpasteurized milk. It crosses the placenta and can cause spontaneous abortion, preterm labor, or stillbirth.
- Toxoplasmosis spreads through undercooked or raw contaminated meat, contact with cat feces (from litter boxes or garden soil), and unwashed fruits or vegetables. A first-time infection during pregnancy is what poses the risk; women who were infected before conception typically have immunity.
- CMV is extremely common and usually harmless in adults, but a new infection during pregnancy transmits to the fetus in 30 to 40% of cases. The most common way women contract it is through contact with the saliva or urine of toddlers who are shedding the virus.
These infections are preventable through food safety practices, hand hygiene, and avoiding specific exposures during pregnancy.
Caffeine, Alcohol, and Smoking
Among lifestyle factors, alcohol consumption during pregnancy carries the clearest risk. One study found that women who consumed five or more units of alcohol per week had nearly five times the odds of miscarriage compared to those who didn’t drink. That’s a strong association, and no amount of alcohol has been established as safe in pregnancy.
High caffeine intake also raises risk, though at a higher threshold than many people assume. The same study found that consuming 375 milligrams or more of caffeine per day (roughly three to four cups of brewed coffee) more than doubled the odds of miscarriage. Most guidelines recommend staying under 200 milligrams per day, which is about one 12-ounce cup of coffee. Interestingly, smoking 10 to 20 cigarettes per day did not show a statistically significant increase in miscarriage risk after adjusting for other factors in that study, though smoking is linked to many other pregnancy complications.
What Doesn’t Cause Miscarriage
Exercise is one of the most common concerns, and the evidence is reassuring. A meta-analysis of 13 randomized controlled trials involving over 3,700 women found no increase in miscarriage risk among those who engaged in low- to moderate-intensity exercise compared to those who didn’t. The authors concluded that pregnant individuals with no medical contraindications can exercise safely throughout pregnancy without fear of miscarriage. Data on very high-intensity or very high-volume exercise (over seven hours per week) is more limited and slightly less clear, but for most women, staying active is safe and even potentially protective.
Sex does not cause miscarriage. Neither does lifting something heavy on a single occasion, working at a computer, or experiencing a minor fall. The guilt many people feel after a miscarriage often centers on something they did or didn’t do, but most early losses are caused by chromosomal errors that no behavior could have prevented.
Recurrent Loss
Having one miscarriage is common and does not mean the next pregnancy will end the same way. Fewer than 5 in 100 women experience two miscarriages in a row. When someone does have three or more consecutive losses before 10 weeks, testing typically looks for chromosomal issues in both parents, uterine abnormalities, thyroid function, blood sugar control, and antiphospholipid antibodies. In many cases of recurrent loss, a specific and treatable cause is found.
Recognizing Early Miscarriage Symptoms
Light bleeding in early pregnancy is fairly common and doesn’t necessarily mean a miscarriage is happening. Brown discharge, which looks like coffee grounds, is often old blood leaving the uterus slowly and is not always a sign of trouble. Pink or light spotting can also occur normally, especially around the time a period would have been due.
The symptoms that more clearly point toward miscarriage include bright red bleeding or clots, heavy bleeding that soaks through a pad, severe abdominal cramping, passage of tissue, or a gush of clear or pink fluid. When a miscarriage is in progress, the bleeding and cramping typically intensify as the pregnancy passes. Some early miscarriages, sometimes called “threatened miscarriages,” start with spotting but the pregnancy continues normally. Until an ultrasound confirms what’s happening, bleeding alone doesn’t give a definitive answer.