Most miscarriages happen because of random genetic errors in the embryo, not because of anything the parent did or didn’t do. About 80% of miscarriages occur in the first 12 weeks of pregnancy, and roughly half of those are caused by chromosomal abnormalities that prevent the embryo from developing normally.
Chromosomal Errors Are the Leading Cause
When a sperm and egg combine, they each contribute 23 chromosomes. Sometimes that process goes wrong, and the embryo ends up with too many or too few chromosomes, a condition called aneuploidy. This typically happens when chromosomes fail to separate properly during the formation of the egg or sperm. The result is an embryo that simply can’t grow into a viable pregnancy, and the body ends it early.
These errors are essentially random. They don’t reflect a problem with either parent’s health or genetics in most cases. However, the chance of these errors does rise with age, particularly the age of the egg. Women aged 25 to 29 have the lowest miscarriage risk at about 10%. By ages 35 to 39, that climbs to roughly 17%. At 40 to 44, it reaches 32%, and after 45, more than half of recognized pregnancies end in miscarriage. This steep rise is driven largely by the increasing likelihood of chromosomal mistakes in older eggs.
In a smaller number of cases, one parent carries a structural chromosomal rearrangement called a balanced translocation. The parent is completely healthy because all their genetic material is present, just reorganized. But when passed to an embryo, the rearrangement can result in missing or extra genetic material, causing repeated losses. This is one reason couples who experience recurrent miscarriage (defined in the U.S. as two or more consecutive losses) are sometimes offered genetic testing.
Hormone and Thyroid Problems
An estimated 8 to 12% of pregnancy losses are linked to endocrine factors, meaning hormone-related conditions. Two of the most significant are thyroid disease and uncontrolled diabetes.
Even mildly low thyroid function can increase the risk of miscarriage and fetal death. Thyroid hormones play a critical role in early pregnancy, supporting the placenta and the embryo’s developing brain. When thyroid output is too low, it disrupts the hormonal environment needed to sustain the pregnancy. What makes this more complicated is that some women have thyroid autoantibodies, immune proteins that attack the thyroid gland, even when their thyroid hormone levels appear normal. About 10% of women carry these antibodies at 14 weeks of pregnancy, and their presence alone is associated with higher rates of pregnancy loss. Researchers believe these antibodies may reflect a broader immune overreaction that targets the placenta itself.
Poorly controlled diabetes creates a similar problem. High blood sugar levels in early pregnancy can interfere with embryo development during the critical weeks when organs are forming. When blood sugar is well managed before and during pregnancy, the added risk drops significantly.
Uterine Shape and Structure
The physical shape of the uterus matters more than many people realize. Some women are born with a uterus that has an unusual shape: a wall of tissue dividing the cavity (a septum), a heart-shaped top (bicornuate), or a smaller-than-normal cavity (unicornuate). These variations can limit the space available for an embryo to implant and grow, or they can restrict blood flow to the developing placenta.
Acquired conditions can cause similar problems. Fibroids that grow into the uterine cavity, polyps, and scar tissue (intrauterine adhesions, sometimes called Asherman syndrome) can all distort the lining where the embryo needs to attach. These structural issues are particularly associated with recurrent pregnancy loss and are often treatable with minor surgical procedures.
Blood Clotting and Immune Disorders
A healthy pregnancy depends on steady blood flow through the placenta. In antiphospholipid syndrome (APS), the immune system produces antibodies that trigger excessive clotting. These antibodies activate blood cells and promote the formation of tiny clots in the placental blood vessels, essentially starving the embryo of oxygen and nutrients. APS can also interfere directly with the placenta’s structure by disrupting a natural anticoagulant that normally protects the placental surface during its formation.
APS is one of the few treatable causes of recurrent miscarriage. It’s uncommon overall, but it’s specifically looked for when someone has experienced multiple losses, because treatment with blood-thinning medication can significantly improve the odds of carrying a pregnancy to term.
Infections That Threaten Pregnancy
Certain infections can cross from a pregnant person to the developing fetus, causing miscarriage, stillbirth, or serious harm. Three of the most well-documented are toxoplasmosis (spread through undercooked meat or contaminated cat litter), listeriosis (a foodborne illness linked to unpasteurized dairy, deli meats, and soft cheeses), and cytomegalovirus, or CMV (a common virus spread through bodily fluids).
Listeria is particularly dangerous during pregnancy because the bacteria can infect the uterine lining and amniotic fluid, leading to spontaneous abortion, preterm labor, or neonatal sepsis. Toxoplasmosis can cause stillbirth and severe neurological damage if the fetus is infected. These infections are preventable with basic food safety and hygiene, which is why dietary guidelines during pregnancy emphasize avoiding high-risk foods.
Caffeine, Alcohol, and Smoking
Lifestyle factors can influence miscarriage risk, though some matter more than others. Drinking five or more units of alcohol per week during pregnancy is associated with nearly five times the odds of miscarriage. Consuming 375 milligrams or more of caffeine daily, roughly equivalent to three to four cups of brewed coffee, is linked to about double the risk. Interestingly, the same research found that smoking 10 to 20 cigarettes per day did not show a statistically significant increase in miscarriage risk after adjusting for other factors, though smoking carries many other serious pregnancy risks.
For caffeine, most major guidelines recommend staying under 200 milligrams per day during pregnancy, which is about one 12-ounce cup of coffee. That threshold is well below the level where research shows a clear increase in risk.
What Doesn’t Cause Miscarriage
Many people who experience a miscarriage search their memory for something they did wrong. The evidence is reassuring on several common fears. A 2023 meta-analysis of 13 randomized trials involving nearly 3,800 women found no increase in miscarriage risk from low- to moderate-intensity exercise during pregnancy. Sex during pregnancy does not cause miscarriage. Working, traveling, and normal daily physical activity are not risk factors.
Stress is more nuanced. It has been listed alongside other factors that may contribute to miscarriage risk, and reducing stress is generally encouraged for overall well-being during pregnancy. But everyday emotional stress is not considered a direct cause. The guilt that often accompanies a miscarriage is, in most cases, misplaced. The single most common cause, chromosomal error, is a biological accident that no behavior can prevent or predict.
When the Cause Is Unknown
Even after thorough evaluation, many miscarriages have no identifiable cause. This is true even for recurrent losses. After testing for chromosomal issues, uterine abnormalities, hormone disorders, and clotting conditions, a significant portion of cases are classified as idiopathic, meaning unexplained. That’s frustrating for anyone searching for answers, but it also means there’s no hidden problem being missed. Most people with unexplained miscarriage go on to have a successful pregnancy without any treatment at all.