Most miscarriages happen because of chromosomal abnormalities in the embryo, not because of anything the pregnant person did or didn’t do. About 50% of all first-trimester miscarriages are caused by the embryo having too many or too few chromosomes, making normal development impossible. Beyond genetics, a range of health conditions, anatomical differences, infections, and environmental exposures can also play a role.
Chromosomal Abnormalities
When an egg or sperm carries an abnormal number of chromosomes, the resulting embryo inherits that error. The embryo may begin developing but cannot sustain a viable pregnancy. These random genetic mistakes are the single most common explanation for early pregnancy loss, responsible for roughly half of all miscarriages in the first 13 weeks. They become more frequent as both egg and sperm age, which is a major reason miscarriage rates climb with parental age.
These errors are almost always sporadic, meaning they happen by chance during cell division rather than being inherited from a parent’s genes. A person who miscarries due to a chromosomal abnormality in one pregnancy can go on to have a chromosomally normal pregnancy the next time.
Maternal Age
Age is one of the strongest predictors of miscarriage risk, and the numbers shift dramatically across the reproductive years. Between ages 20 and 30, the chance of miscarriage sits between 9% and 17%. At 35, it rises to about 20%, or 1 in 5 pregnancies. By 40, the risk doubles to 40%. And at 45, roughly 80% of pregnancies end in miscarriage. Most of this increased risk traces back to the higher rate of chromosomal errors in eggs as a person ages.
Paternal Age and Sperm Quality
The father’s age matters too, though it gets far less attention. Men over 40 show significantly higher levels of DNA damage in their sperm compared to younger men. One study found that sperm DNA fragmentation averaged 15% in men over 40, compared to 9% in younger men. When DNA fragmentation exceeds 20%, the risk of miscarriage climbs noticeably. Older fathers also show lower levels of a chemical process called methylation that helps genes function properly, dropping from 6% in younger men to 2% in those over 40. These changes in sperm quality can impair embryo development even when the egg is healthy.
Hormonal and Chronic Health Conditions
Several underlying health conditions increase the likelihood of miscarriage, particularly when they’re unmanaged during early pregnancy.
Thyroid disorders, both overactive and underactive, are common in pregnant people and have been directly linked to miscarriage and premature birth. The thyroid regulates metabolism and energy use throughout the body, and an imbalance can disrupt the hormonal environment the embryo needs to implant and grow.
Uncontrolled diabetes is another significant risk factor. High blood sugar levels in early pregnancy can interfere with fetal development before many people even know they’re pregnant. The key word is “uncontrolled.” People with well-managed diabetes generally have much better pregnancy outcomes, which is why blood sugar optimization before conception makes a real difference.
Uterine and Structural Differences
The shape and condition of the uterus plays a direct role in whether a pregnancy can be sustained. A septate uterus, where a wall of tissue partially or fully divides the uterine cavity, is one of the most well-studied structural causes. In one study, 42% of pregnancies in people with a septate uterus ended in first-trimester miscarriage, compared to 12% in people without a uterine anomaly. A large meta-analysis found that a septate uterus nearly triples the risk of both first-trimester and second-trimester pregnancy loss. Surgical correction of the septum has been shown to reduce miscarriage rates significantly.
Uterine fibroids, which are noncancerous growths in or on the uterine wall, can also interfere with implantation or blood supply to the embryo depending on their size and location. Cervical insufficiency, where the cervix opens too early under the weight of the growing pregnancy, is another structural issue that typically causes losses in the second trimester rather than the first.
Immune System Disorders
Antiphospholipid syndrome (APS) is one of the few treatable causes of recurrent miscarriage. It’s an autoimmune condition where the body produces antibodies that promote abnormal blood clotting. These clots can form in the tiny blood vessels of the placenta, cutting off nutrient and oxygen supply to the developing embryo. Estimates suggest APS is the primary risk factor in 7% to 25% of recurrent miscarriages. The wide range reflects differences in how studies define recurrent loss and measure the relevant antibodies, but the connection itself is well established. Once diagnosed, APS can often be managed with blood-thinning medications during pregnancy.
Infections
Certain bacterial, viral, and parasitic infections can directly cause miscarriage or increase the risk substantially. Listeria is one of the most dangerous. It’s a type of bacteria found in refrigerated, ready-to-eat foods like deli meats, soft cheeses, and smoked seafood. A pregnant person with listeriosis may feel only mild flu-like symptoms, or nothing at all, yet the infection can lead to miscarriage, stillbirth, or premature delivery.
Toxoplasmosis, caused by a parasite commonly found in undercooked meat and cat feces, poses similar risks. Other infections linked to pregnancy loss include rubella, cytomegalovirus, and certain sexually transmitted infections when left untreated. The unifying factor is that these pathogens can cross the placenta or trigger an inflammatory response that disrupts the pregnancy.
Environmental and Chemical Exposures
Exposure to certain toxins in the environment or workplace is associated with both infertility and miscarriage. Lead-contaminated water and methylmercury (which accumulates in certain fish) are two of the best-documented chemical risks. People working in agriculture, manufacturing, dry cleaning, beauty salons, custodial services, and healthcare may face higher exposure to pesticides, organic solvents, heavy metals, phthalates, and radiation. Levels of pesticides and phthalates measured in occupationally exposed workers are far higher than those found in the general population, making workplace exposure a genuine concern rather than a theoretical one.
Lifestyle Factors That Do and Don’t Matter
Smoking and heavy alcohol use during pregnancy are clearly linked to increased miscarriage risk. Both reduce blood flow to the placenta and can directly damage developing cells. Illicit drug use, particularly cocaine and methamphetamines, carries similar risks.
Caffeine falls into a more nuanced category. Moderate consumption, defined as less than 200 milligrams per day (roughly one 12-ounce cup of coffee), does not appear to be a major contributing factor in miscarriage or preterm birth. That threshold has been consistently maintained in clinical guidance reaffirmed as recently as 2023.
What doesn’t cause miscarriage often matters just as much to people searching for answers. Moderate exercise, sexual intercourse, and short-term everyday stress have not been shown to raise miscarriage risk. The Mayo Clinic states plainly that brief periods of stress that don’t disrupt a person’s overall life don’t seem to increase the chance of pregnancy loss. Most of the time, there is nothing a pregnant person does that causes a miscarriage. That reality can be difficult to internalize after a loss, but the evidence behind it is strong.