What Does and Doesn't Cause a Miscarriage

People hear a lot of things about what does and does not cause miscarriages, and much of the information available is confusing at best and harmful at worst. In fact, it's estimated that as many as 26% of all pregnancies end in miscarriage with the likelihood increasing with advancing age.

"Much misinformation is shared among women or passed down from older generations," says Jonathan Schaffir, MD, an assistant professor of obstetrics and gynecology at Ohio State University College of Medicine.

What research reveals are miscarriage risk factors, or things that are correlated with miscarriage and may increase a person's risk of experiencing one. Some common causes of miscarriage is through risk factors such as drug and alcohol abuse, genetic conditions, infections, uterine abnormalities, cervical abnormalities, and more. Here's what you should know about what causes a miscarriage—and what does not.

What Causes Miscarriage

The majority of miscarriages, especially in the first trimester, are caused by chromosomal abnormalities in the developing embryo or fertilized egg, explains the American College of Obstetricians and Gynecologists (ACOG). Beyond chromosomal abnormalities, there are other known causes of miscarriage.

Despite all that's known about what can cause miscarriage, there is still a lot that experts don't know. Even after a fertility evaluation for pregnancy loss, up to 50% of miscarriages are attributed to unknown causes.

Because the majority of people who conceive again after experiencing a miscarriage go on to have a healthy pregnancy, it's common to remain in the dark about what caused the miscarriage. Things may look a little different when you're dealing with recurrent miscarriage (two or more back-to-back pregnancy losses), at which point a health care provider may be more likely to recommend doing some testing to pinpoint a possible cause and determine the best course of action.

The Most Common Causes of Miscarriage

While it may be hard to pinpoint an exact reason behind a pregnancy loss, here's what experts do know about the most common causes of miscarriage and some of the factors that may increase a person's risk of having one.

Chromosomal abnormalities in the fetus

Research suggests that at least 50% of first-trimester miscarriages are caused by chromosomal abnormalities in the fetus.

"When the chromosomes of the egg and those of the sperm fuse to form an embryo, they usually pair up correctly," says Henry Lerner, MD, an OB-GYN at Newton-Wellesley Hospital in Newton, Massachusetts. "But sometimes they get scrambled; if they're paired incorrectly, the embryo stops developing."

Chromosomal abnormalities don't mean that anything's wrong with the parents or their genes, and because chromosomal abnormalities in embryos are often random, one-time events, they're unlikely to happen again.

Certain health conditions in the pregnant person

Certain health conditions can cause miscarriage, especially those that restrict blood flow to the uterus. Chronic conditions that are associated with a higher risk of miscarriage include hypertension, diabetes, thyroid disease, lupus, and heart disease.

Hormone-related disorders and autoimmune disorders like antiphospholipid syndrome (APS) have also been linked to miscarriage. Certain types of bacterial and viral infections like uterine infections and sexually transmitted infections (STIs) can also play a role. Examples include:

• Fifth disease
• Rubella (German measles)
• Bacterial vaginosis
• Chlamydia
• Listeria

Problems with the uterus or cervix, such as fibroids and cervical insufficiency (when the cervix dilates too soon during pregnancy), can also lead to miscarriage.

Other health-related factors include certain medical procedures such as amniocentesis and chorionic villus sampling (CVS), which carry a very small risk of miscarriage (one of the reasons why these procedures are reserved for cases where the benefits outweigh the risks).

Excess caffeine consumption

Caffeine is one drug that comes up a lot in the context of pregnancy. Let's be clear: One daily cup coffee during pregnancy is perfectly fine. It's when caffeine consumption gets higher that there may be a risk.

Much of the conversation about caffeine and miscarriage is based on a 2008 study that found that people who consumed 200 milligrams or more of caffeine a day had twice the miscarriage risk as those who didn't have any.

Since then, additional studies have demonstrated a connection between excessive caffeine consumption and miscarriage, but there isn't consensus about the nature and strength of the connection. The official stance of the ACOG is that it's safe for pregnant people to drink up to 200 mg of coffee per day, which is the amount of caffeine in about two 8-ounce cups of regular coffee.

Drug and alcohol use

While the research about caffeine and miscarriage risk is incomplete, the evidence is clear that drug and alcohol use can lead to miscarriage. "Exposing a fetus to large amounts of these chemicals on a regular basis can cause miscarriage because they have a poisonous effect on all those developing cells," says Dr. Schaffir. Substances that can lead to miscarriage include alcohol, illicit drugs, nicotine and tobacco products, and even certain prescription medications.

Researchers note that the risk of stillbirth is raised by 1.8 to 2.8 times with tobacco use, 2.3 times with marijuana use, 2.2 times with stimulants or prescription drug use, and even 2.1 times greater risk of miscarriage from passive exposure to tobacco.

Hot tubs and high body temperatures

Hot tub use during pregnancy may increase the risk of miscarriage, according to a 2003 study. In that study, the risk of miscarriage was doubled on average with early first-trimester hot tub use and increased further with greater frequency of use.

The problem with hot tubs, saunas, and hot baths isn't the devices themselves but rather the increase in body temperature associated with their use. Allowing your body temperature to get too high during pregnancy has also been linked with neural tube defects and is not recommended.

What Doesn't Cause Miscarriage

While we know what things can cause or increase the risk of miscarriage, there are still plenty of myths surrounding miscarriage. We asked Dr. Schaffir to debunk some major misconceptions about miscarriages.

"It's important for [people] to understand that these are just old wives' tales—and not only are they not true but in some cases, believing them can affect your health and well-being," he says. Here are four things that don't cause a miscarriage.

Moderate exercise

Exercising or picking up a (reasonably) heavy object—such as a grocery bag, a toddler, or the like—are extremely unlikely to cause a miscarriage. In fact, the ACOG notes that exercise during pregnancy, with your doctor's approval, can have benefits during pregnancy.

That's because exercise reduces stress, relieves aches and pains, lowers your risk of pregnancy-related conditions like gestational diabetes, gestational hypertension, and preeclampsia, improves your overall fitness, and even builds up stamina for labor. As long as you're continuing exercises that you did pre-pregnancy and not suddenly taking on Olympic weightlifting, exercise during pregnancy is perfectly safe.

Abortion

While there are a couple of studies that have suggested an increased risk of miscarriage in people who had previously terminated a pregnancy, the evidence overwhelmingly indicates that elective abortion is safe and not linked to future pregnancy loss. In fact, a large study published in The New England Journal of Medicine found that elective medical abortion was not associated with any increased risk of subsequent miscarriage.

Any increased risk of future miscarriage appears to be limited to people who experienced post-surgery complications such as severe infection or uterine scarring after a surgical abortion via dilation and curettage (D&C). And there is no evidence that medical abortion, or an abortion using medications rather than surgery, is associated with future miscarriages.

Everyday stress

While some studies on stress and miscarriage are conflicting, Dr. Schaffir says that everyday tension or anxiety—such as tight deadlines at work or worrying about what labor will be like—have not been linked to pregnancy loss. What's more, no studies have ever linked excessive bad moods to miscarriage, Dr. Schaffir adds.

Things get murky when dealing with major stress, though. "We're talking big things, like the death of a spouse or parent," he explains, and even then, the link is not well established. Plus, pregnant people who are under extreme stress may be more likely to smoke, drink, or do drugs to cope—which can also affect their risk of miscarrying.

Food that's properly cooked

While foods themselves don't cause miscarriage, certain foods come with higher risks of food-borne illnesses like listeriosis (an infection caused by the bacteria Listeria monocytogenes).

The Food and Drug Administration (FDA) explains that cases of listeriosis and other food-borne illnesses have been linked to miscarriage and pregnancy complications, which is why experts recommend avoiding certain foods during pregnancy.

For instance, pregnant people should avoid raw meat and fish, unpasteurized cheeses, and deli meat, all of which carry a higher risk of food-borne illness. Not only are you more likely to develop food poisoning while pregnant, but some of the organisms that cause food poisoning are linked with an increased risk of miscarriage. These include:

• Listeria
• Salmonella
• E. coli
• Toxoplasmosis

It's important to note that you don't have to avoid all cheese or deli meats during pregnancy.

Sex

In general, all kinds of sex and sex acts including the use of sex toys are considered safe during pregnancy. The fetus is protected not only by amniotic fluid but also by the powerful muscles of the uterine walls. As long as the pregnancy is low risk, the fetus is developing on a typical track, and there are no concerns about pre-term labor or other complications, sex during pregnancy is likely safe. If you're concerned, however, don't hesitate to ask a health care provider.

Is Miscarriage Ever the Pregnant Person's Fault?

The vast majority of miscarriages occur because of chance chromosomal or genetic abnormalities in the embryo or fetus or, less commonly, hormonal imbalances or problems with the uterus or placenta, says Dr. Schaffir.

These factors are nothing that a pregnant person has control over. It's natural for people experiencing loss to try to explain it in some way, even if that means blaming themselves. "But all [people] need to know that most of the time, a miscarriage is completely random," he says.

Dr. Schaffir adds that if you eventually want to try again, odds are you will conceive and have a healthy pregnancy. Of course, that being said, if you have any concerns, have had two or more losses in a row, or are over the age of 35, it's always a good idea to speak with a doctor who can help you plan your next steps.