About 15% of known pregnancies end in miscarriage, and the single biggest cause is a genetic problem in the embryo that happens by chance. Beyond that, a range of medical conditions, infections, structural issues, and environmental exposures can raise the risk. Importantly, everyday activities like exercise, sex, and work do not cause miscarriage.
Chromosomal Problems Cause Most Early Losses
Somewhere between 40% and 76% of first-trimester miscarriages are caused by chromosomal abnormalities in the embryo. These are random errors that occur when cells divide in the earliest stages of development, resulting in an embryo with too many or too few chromosomes. The pregnancy simply cannot develop normally, and the body ends it early. This is by far the most common explanation for a single miscarriage, and it generally has nothing to do with the health or behavior of either parent.
Maternal Age and Miscarriage Risk
Age is one of the strongest predictors of miscarriage, largely because chromosomal errors in eggs become more frequent over time. The numbers shift significantly 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 apply to known pregnancies. Many very early losses happen before a person even realizes they’re pregnant, which means the true miscarriage rate at every age is likely higher.
Chronic Health Conditions
Several ongoing medical conditions can interfere with a pregnancy if they aren’t well controlled. Diabetes is a major one. When blood sugar levels stay elevated during early pregnancy, it creates a hostile environment for a developing embryo. The risk is tied to uncontrolled blood sugar, not the diagnosis itself. People with well-managed diabetes have much better pregnancy outcomes.
Thyroid disorders also play a significant role. Both an overactive and underactive thyroid have been linked to miscarriage, premature birth, and complications for the baby. High thyroid hormone levels during pregnancy are additionally tied to gestational diabetes, which carries its own risks. Thyroid problems are common among pregnant women and are one of the more treatable causes of pregnancy loss when caught early.
Other conditions that raise miscarriage risk include polycystic ovary syndrome (PCOS), kidney disease, and clotting disorders.
Antiphospholipid Syndrome
Antiphospholipid syndrome (APS) is an autoimmune condition that causes recurrent pregnancy loss. For a long time, doctors assumed APS caused miscarriage by forming blood clots in the placenta. But newer research tells a different story. Clots are not commonly found in miscarriage tissue from APS patients. Instead, the antibodies produced in APS appear to directly attack the placenta’s surface, disrupting the cells responsible for establishing and maintaining the pregnancy. This triggers inflammation at the boundary between mother and fetus, interfering with normal placental development. APS is one of the most important treatable causes of recurrent miscarriage, and testing for it is routine after two or three consecutive losses.
Uterine and Cervical Problems
Structural issues in the uterus or cervix can cause pregnancy loss, particularly in the second trimester. A septate uterus, where a wall of tissue divides the uterine cavity, can prevent an embryo from implanting properly or restrict its growth. Uterine fibroids, depending on their size and location, can also interfere.
Cervical insufficiency is when the cervix opens or weakens too early, typically between weeks 14 and 27. It accounts for nearly 25% of second-trimester miscarriages. Risk factors include previous cervical surgeries, damage from a prior delivery, or being born with an irregularly shaped cervix. Unlike most first-trimester losses, cervical insufficiency often has no warning signs until the cervix has already begun to dilate. When identified in time, a stitch can be placed to hold the cervix closed for the remainder of the pregnancy.
Infections That Threaten Pregnancy
Certain infections can cross the placenta or disrupt pregnancy in other ways. The ones most clearly linked to miscarriage and pregnancy loss include:
- Listeria: Found in unpasteurized dairy, deli meats, and some ready-to-eat foods. Infection during pregnancy can lead to miscarriage, stillbirth, or preterm birth.
- Gonorrhea: Untreated infection has been linked to miscarriage, preterm birth, low birth weight, and premature rupture of membranes.
- Syphilis: Connected to preterm birth, stillbirth, and in some cases, death shortly after birth.
- Zika virus: Can cause pregnancy loss, birth defects, and other complications.
- Parvovirus B19 (fifth disease): Usually harmless in pregnancy, but in rare cases can cause miscarriage or severe anemia in the fetus.
Most of these are either preventable through food safety and hygiene or detectable through routine prenatal screening. Sexually transmitted infections in particular are tested for early in pregnancy precisely because treatment can prevent complications.
Caffeine and Substance Use
Caffeine is one of the few dietary factors with consistent evidence behind it. A large prospective study found that consuming 200 milligrams or more of caffeine per day (roughly two standard cups of coffee) was associated with nearly double the risk of miscarriage compared to no caffeine. Even amounts below 200 mg per day showed a smaller but measurable increase in risk. This is why most guidelines suggest keeping caffeine under 200 mg daily during pregnancy.
Smoking, alcohol, and recreational drugs all raise miscarriage risk as well. Smoking restricts blood flow to the placenta. Alcohol can directly damage a developing embryo. Cocaine and methamphetamine are particularly dangerous because they cause sudden spikes in blood pressure and can reduce blood supply to the uterus.
Environmental and Occupational Exposures
Exposure to heavy metals before or during pregnancy can increase the risk of miscarriage, stillbirth, and birth defects. The CDC specifically identifies lead, cadmium, cobalt, mercury, and nickel as reproductive hazards. People who work in manufacturing, battery production, painting, soldering, or certain agricultural settings may have higher occupational exposure. These metals can also disrupt hormones and reduce fertility even before conception. If your job involves contact with heavy metals or chemical solvents, workplace protections and exposure limits become especially important during pregnancy.
What Does Not Cause Miscarriage
One of the most persistent and damaging beliefs about miscarriage is that something the pregnant person did caused it. Exercise, lifting something heavy, having sex, working long hours, getting into an argument, feeling stressed: none of these cause miscarriage. As one maternal-fetal medicine specialist at Brigham & Women’s Hospital put it, “It’s extremely hard to cause your own miscarriage.” The guilt that follows a loss is almost universal, and almost always unfounded. The vast majority of miscarriages happen because of chromosomal problems or medical conditions that were already present, not because of anything that could have been done differently.