About half of all miscarriages in the first trimester are caused by chromosomal abnormalities in the embryo, making genetic problems the single most common cause of pregnancy loss. Beyond genetics, a range of health conditions, lifestyle factors, and environmental exposures can also play a role. Most miscarriages happen before 13 weeks of pregnancy, and in many cases, nothing could have been done to prevent them.
Chromosomal Abnormalities
Chromosomal abnormalities account for roughly 50% of all first-trimester miscarriages. These are random errors that occur when the fertilized egg divides and ends up with too many or too few chromosomes. The resulting embryo simply cannot develop normally, and the pregnancy ends on its own.
These errors are not inherited conditions in most cases. They happen by chance during cell division, which is why they become more frequent as egg quality declines with age. There is no way to prevent them, and experiencing one does not mean the next pregnancy will have the same problem.
How Age Affects Risk
Maternal age is one of the strongest predictors of miscarriage, largely because chromosomal errors in eggs become more common over time. The numbers shift dramatically across age groups:
- Ages 20 to 30: 9% to 17% chance of miscarriage
- Age 35: about 20% (1 in 5)
- Age 40: about 40% (4 in 10)
- Age 45: about 80% (8 in 10)
These numbers reflect the overall likelihood of pregnancy loss at each age, not a guarantee. Many people over 35 carry healthy pregnancies to term. But the steep rise after 40 explains why age is a factor fertility specialists take seriously when evaluating recurrent losses.
Chronic Health Conditions
Several underlying health problems can increase the risk of miscarriage if they aren’t well managed before and during pregnancy.
Uncontrolled diabetes is one of the most significant. High blood sugar levels in early pregnancy can interfere with the embryo’s development during the critical first weeks, sometimes before a person even knows they’re pregnant. Diabetes that is well controlled before conception carries a much lower risk.
Thyroid disorders, both overactive and underactive, have been linked to miscarriage, premature birth, and other complications. The thyroid helps regulate hormones essential to sustaining a pregnancy, so imbalances can disrupt the process at multiple stages. Screening and treatment before or early in pregnancy can significantly reduce this risk.
Polycystic ovary syndrome (PCOS), high blood pressure, and kidney disease can also contribute to pregnancy loss, though the mechanisms vary. In general, the better a chronic condition is managed going into pregnancy, the lower the risk.
Blood Clotting and Immune Disorders
Antiphospholipid syndrome (APS) is an autoimmune condition that causes the blood to clot too easily. During pregnancy, those clots can block blood flow through the placenta, cutting off the nutrient supply the embryo needs to survive. APS can also raise the risk of preeclampsia.
Many people don’t know they have APS until after a miscarriage or an unexplained blood clot. If you’ve had recurrent pregnancy losses, testing for APS is one of the first things a provider will consider. The condition is treatable during pregnancy, typically with blood thinners and low-dose aspirin, which can dramatically improve the chances of carrying to term.
Other inherited clotting disorders can pose similar risks by compromising placental blood flow, though APS is the most well-studied in relation to recurrent miscarriage.
Uterine Structure
The physical shape of the uterus matters. A uterine septum, a wall of tissue that partially or fully divides the uterine cavity, is associated with an increased risk of recurrent miscarriage. Larger septa tend to cause more problems than smaller ones. The septum may interfere with proper implantation or restrict the space available for the embryo to grow.
Fibroids, which are noncancerous growths in the uterine wall, can also contribute to pregnancy loss depending on their size and location. Fibroids that grow into the uterine cavity or distort its shape are more concerning than those on the outer wall. Both septums and fibroids can often be identified with imaging and treated surgically before a future pregnancy.
Infections
Certain infections pose a direct threat to pregnancy. Listeria, a type of bacteria found in unpasteurized dairy, deli meats, and some ready-to-eat foods, is particularly dangerous. One in four pregnant women who develop a listeria infection lose their pregnancy or their baby shortly after birth, even if the mother’s own symptoms are mild.
Toxoplasmosis, typically contracted from undercooked meat or contact with cat feces, can cause pregnancy loss or serious birth defects if the infection occurs during pregnancy. Rubella (German measles) is another well-known risk, though widespread vaccination has made it rare in many countries. Sexually transmitted infections like untreated syphilis and certain bacterial infections of the reproductive tract can also contribute to miscarriage.
Environmental and Chemical Exposures
Pesticide exposure has been linked to a 41% higher risk of miscarriage, according to a large meta-analysis covering over 439,000 participants. Pesticides appear to cause harm through inflammation, oxidative stress, and disruption of the hormonal systems that support pregnancy. This risk applies most to people with occupational exposure (agricultural workers, for example), though household pesticide use may also contribute.
Lead, mercury, and certain industrial solvents have also been associated with pregnancy loss. High-dose radiation exposure is a known risk. For most people, the practical concern is avoiding unnecessary chemical exposures during pregnancy, particularly in workplaces where contact with industrial chemicals or heavy metals is routine.
Caffeine, Alcohol, and Smoking
Moderate caffeine consumption, defined as less than 200 mg per day (roughly one 12-ounce cup of coffee), does not appear to significantly increase miscarriage risk. The American College of Obstetricians and Gynecologists considers this level safe. Going above that threshold has been associated with a modest increase in risk in some studies, though findings are mixed.
Alcohol and smoking are more clearly linked to pregnancy loss. Smoking reduces blood flow to the placenta and exposes the embryo to toxic chemicals. Alcohol, particularly heavy or binge drinking, can directly harm fetal development. There is no established safe level of alcohol during pregnancy, which is why complete avoidance is the standard recommendation. Recreational drug use, especially cocaine and methamphetamine, also significantly raises miscarriage risk.
What Doesn’t Cause Miscarriage
Exercise, sex, and everyday stress are not causes of miscarriage, despite widespread concern. Most studies show no connection between physical activity and pregnancy loss. Moderate exercise like walking or swimming may actually reduce the risk of later complications such as gestational diabetes. Even strenuous workouts and weightlifting are typically safe for physically fit women with healthy pregnancies.
Sexual intercourse does not increase the risk of early miscarriage. Working, lifting normal household objects, arguments, emotional upset, and minor falls also do not cause pregnancy loss. These misconceptions can add guilt to an already painful experience, but the evidence is clear: the vast majority of miscarriages result from factors entirely outside a person’s control.