About 15 to 20 percent of clinically recognized pregnancies end in miscarriage, and more than 80 percent of those losses happen in the first trimester. Some risk factors are beyond your control, like age and genetics, while others involve health conditions, lifestyle choices, or exposures you can modify. Here’s what the evidence says about each one.
Chromosomal Abnormalities
Roughly half of all first-trimester miscarriages are caused by chromosomal problems in the embryo. These are random errors that occur when cells divide during early development, not inherited conditions passed down from parents. In a Greek study that analyzed 198 miscarriage samples, about 42 percent had detectable chromosomal abnormalities, and 90 percent of those were numerical errors (an extra or missing chromosome rather than a structural defect).
The most common error was an extra copy of a single chromosome, called autosomal trisomy, which accounted for over half the abnormal cases. Chromosomes 16 and 22 were the most frequently affected. Turner syndrome, where a female embryo has only one X chromosome instead of two, made up about 11 percent of the abnormalities. These errors are largely random and become more likely as egg quality declines with age.
Maternal Age
Age is one of the strongest predictors of first-trimester loss, primarily because older eggs are more prone to the chromosomal errors described above. A large Norwegian registry study tracking pregnancies between 2009 and 2013 mapped the risk curve clearly:
- Under 20: 15.8 percent
- 25 to 29: 9.8 percent (the lowest risk window, bottoming out at age 27)
- 35 to 39: 16.7 percent
- 40 to 44: 32.2 percent
- 45 and older: 53.6 percent
The rise after 35 is gradual at first, then steepens considerably after 40. By 45, the odds of miscarriage are roughly equal to the odds of carrying to term.
Body Weight and Obesity
A BMI of 30 or higher is associated with a 20 to 45 percent increase in first-trimester miscarriage risk compared to women with a normal BMI. The association is strongest in very early pregnancy: one study of over 5,000 women found that the risk of loss before eight weeks was 34 percent higher among obese women. For women with a BMI above 35, the numbers are more striking. One study found an 11.3 percent miscarriage rate in that group compared to 2.7 percent among women with a normal BMI.
Excess body fat affects hormone levels, inflammatory markers, and the uterine environment in ways that can interfere with implantation and early embryo development. Women who experience recurrent first-trimester losses are 3.5 times more likely to be obese than women with normal weight, according to a Danish case-control study.
Thyroid Disease and Chronic Conditions
Even mild, subclinical hypothyroidism (an underactive thyroid that hasn’t yet produced obvious symptoms) increases the risk of miscarriage and fetal death. Thyroid hormones play a direct role in early placental development, so untreated deficiency can disrupt pregnancy before many women even know they have a thyroid problem. Treatment with thyroid hormone replacement reduces this risk.
Uncontrolled diabetes, particularly elevated blood sugar levels during the weeks surrounding conception and early embryo development, is another well-established risk factor. The key word is “uncontrolled.” Women with diabetes who maintain tight blood sugar management before and during early pregnancy bring their miscarriage risk much closer to the baseline.
Alcohol Consumption
A meta-analysis of data from nearly 232,000 pregnant women found that any alcohol use during pregnancy raised miscarriage risk by about 19 percent compared to abstaining. The relationship is dose-dependent: for women drinking five or fewer drinks per week, each additional drink per week increased the risk by 6 percent. When researchers used survival data that tracked timing more precisely, each additional weekly drink was associated with a 13 percent increase in the hazard of miscarriage.
There is no established “safe” threshold. Because the risk increases in a linear, dose-dependent fashion starting from low levels of consumption, the clearest takeaway is that less is better and none is safest.
Caffeine
Caffeine intake of 200 mg per day or more (roughly two standard cups of coffee) is linked to a doubled risk of miscarriage, according to research cited by the American College of Obstetricians and Gynecologists. Below 200 mg per day, studies have not found a significant increase in risk. That 200 mg benchmark is why most prenatal guidelines suggest keeping caffeine under that threshold. Keep in mind that caffeine adds up across sources: coffee, tea, chocolate, energy drinks, and some sodas all contribute.
NSAIDs and Pain Medications
Nonsteroidal anti-inflammatory drugs like ibuprofen and naproxen carry a meaningful risk during early pregnancy. A population-based study found that NSAID use during pregnancy increased miscarriage risk by 80 percent overall. The timing and duration mattered enormously: use around the time of conception raised the risk more than fivefold, and use lasting longer than one week was associated with an eightfold increase.
Aspirin showed a similar, though less statistically certain, association. Acetaminophen (Tylenol), which works through a different mechanism, showed no increased risk regardless of timing or duration. If you need pain relief during early pregnancy, acetaminophen is generally considered the safer option.
Uterine Structural Abnormalities
A septate uterus, where a wall of tissue partially divides the uterine cavity, is the structural abnormality most strongly linked to first-trimester loss. Women with a septate uterus have a first-trimester miscarriage rate of about 42 percent, compared to 12 percent in women with a normal uterine cavity. The septum has a poor blood supply, so an embryo that implants on it may not get adequate nourishment. Importantly, a septate uterus specifically increases first-trimester loss rather than later complications like preterm labor.
Other structural variations, including a bicornuate (heart-shaped) uterus and certain types of fibroids, can also affect pregnancy retention, though the evidence is less consistent than for a septate uterus.
Infections
Several infections during early pregnancy are linked to increased miscarriage risk, though the strength of evidence varies. Cytomegalovirus (CMV), a common virus that most adults carry without symptoms, can replicate in the cells that form the placenta, triggering inflammation and cell death that disrupts implantation. A CMV infection occurring around the time of conception poses the highest risk.
Toxoplasmosis, caused by a parasite found in undercooked meat and cat feces, has been associated with higher miscarriage rates in multiple studies, though the data is not entirely consistent. In one study of women who miscarried, 55 percent tested positive for past exposure to the parasite. Listeria, a foodborne bacterium found in unpasteurized dairy, deli meats, and some ready-to-eat foods, can cross from the intestinal lining into the bloodstream and invade the placenta directly. This is why dietary precautions around soft cheeses and cold deli meats are standard prenatal advice.
Smoking
Smoking during early pregnancy reduces blood flow to the developing placenta and exposes the embryo to carbon monoxide and nicotine, both of which impair oxygen delivery. It is a consistently identified risk factor for first-trimester loss across large population studies. The risk is dose-dependent, meaning heavier smoking carries greater risk, and quitting before or during early pregnancy reduces it.
What You Can and Cannot Control
The single largest cause of first-trimester miscarriage, chromosomal abnormality, is essentially random. Age compounds this risk but isn’t something you can change on a timeline. These two factors alone account for the majority of early losses, which is why miscarriage is common even among healthy women doing everything “right.”
The modifiable factors, including alcohol, caffeine over 200 mg, smoking, NSAID use, untreated thyroid disease, uncontrolled blood sugar, and obesity, each contribute a smaller but real share of overall risk. Addressing them before or early in pregnancy can meaningfully lower your chances, even if it can never eliminate the baseline risk entirely.