9 Possible Causes of Miscarriage

While miscarriage is common—with an estimated 1 in every 10 pregnancies ending in a loss—going through it yourself can be a completely overwhelming experience with long-lasting emotional repercussions. The first question that many people ask after experiencing a miscarriage is simple: Why? What caused it?

In many cases, particularly with early miscarriages, it can be hard to determine exactly what went wrong. But as the experts see it, it's actually far less common for pregnancy to actually go right.

"When you think about a pregnancy, and you think about the beginnings of a human being forming and all the things that have to go perfectly, it really and truly is a miracle when it happens," says Elizabeth Nowacki, D.O., an OB-GYN at St. Vincent Fishers Hospital in Indiana. "You have two sets of genetic material coming together that have to divide, and sometimes things go wrong. The simplest way to think about it is that (miscarriage) is sort of nature's way of making sure that a human being is compatible with life."

While many people who experience a miscarriage blame themselves, the truth is that they probably did nothing to cause it. Here's everything you need to know about why miscarriages happen and what causes them.

Types of Miscarriage

A miscarriage is the spontaneous loss of a pregnancy before the 20th week of pregnancy. Miscarriage can happen in several ways, and depending on the type, a person may experience different symptoms. Additionally, the "type" of miscarriage is often defined by what point in the process the miscarriage is caught and diagnosed. Here are five types of miscarriage:

• Missed miscarriage: A missed miscarriage is a pregnancy loss in which the embryo has died, but the pregnant person's body hasn't yet expelled the contents of the uterus. Missed miscarriage often has no symptoms, and the pregnant person may be unaware of it until it is diagnosed by ultrasound.
  
• Threatened miscarriage: Threatened miscarriage is when the cervix remains closed but there is cramping and vaginal bleeding. These symptoms alone do not mean the pregnancy has been lost, and a health care provider will likely monitor closely for any further symptoms.
• Inevitable miscarriage: An inevitable miscarriage is defined by vaginal bleeding and an open (dilated) cervix. When diagnosed with an inevitable miscarriage, your body likely hasn't yet released the pregnancy tissue, but the loss is imminent and cannot be prevented.
• Complete miscarriage: A complete miscarriage is when a pregnancy has been lost, all tissue has been passed, and the uterus is empty.
• Recurrent miscarriage: Recurrent miscarriage is when a person experiences three consecutive miscarriages, which is rare, affecting roughly 1% of pregnant people.

What Causes Miscarriage?

While there isn't always a known reason for a miscarriage, experts do know what things can lead to miscarriage or impact your risk of miscarriage.

The risk of pregnancy loss is highest in the first 12 weeks (first trimester) of pregnancy with an estimated 1 in 10 known pregnancies ending in miscarriage. In fact, about 80% of all miscarriages happen in the first trimester. But the risk of miscarriage decreases significantly after week 12 with just 1 in 100 to 5 in 100 pregnancies ending in miscarriage in the second trimester.

The most common signs and symptoms of miscarriage include vaginal bleeding, cramping, passing tissue from the vagina, and no longer feeling pregnancy symptoms.

Several factors can cause a miscarriage, including genetic abnormalities, thyroid disorders, diabetes, immunological disorders, drug use, and more. Read on to learn about nine of the most common causes of miscarriage in early pregnancy, including additional miscarriage risk factors.

Chromosomal abnormalities

According to the American College of Obstetricians and Gynecologists (ACOG), the most common cause of miscarriage is a genetic abnormality in the embryo. That means that somewhere along the line, the developing embryo had a "glitch" that was incompatible with life, so the embryo stops developing. During fertilization, the sperm and egg each bring 23 chromosomes together to create perfectly matched pairs. This is a complex process, and a minor glitch can result in a genetic or chromosomal abnormality.

While some chromosomal abnormalities are compatible with life (such as trisomy 21, the most common type of Down syndrome), other chromosomal disorders are simply not. In these cases, "genetically, (development) just stops," explains Dr. Nowacki.

Miscarriages caused by chromosomal abnormalities happen more often in people with reproductive abilities who are older than 35. "This is because all the eggs that a woman will ever have are present from birth, and the eggs age with her," says Stephanie Zobel, M.D., an OB-GYN with Winnie Palmer Hospital.

"Paternal age may also similarly play a role," she adds. "The frequency of miscarriage in women below age 20 is around 12% to 15% and doubles as the woman approaches age 40. There is nothing that can be done to prevent miscarriage due to a chromosomal abnormality, and once a miscarriage has begun there is nothing one can do to stop it."

Thyroid disorders

Whether it be hypothyroidism (underactive thyroid) or hyperthyroidism (overactive thyroid), thyroid disorders can lead to problems with infertility and even cause recurrent miscarriages. The Malpani Infertility Clinic's website explains that in cases where a potential pregnant person's thyroid function is low, their body will try to compensate by producing hormones that can actually suppress ovulation. Conversely, a thyroid that produces too many hormones can interfere with estrogen's ability to do its job, and it may make the uterus unfavorable for implantation or lead to abnormal uterine bleeding.

Diabetes

While diabetes itself may not cause a miscarriage, uncontrolled diabetes can lead to complications that could contribute to a miscarriage. "People with diabetes need to work with their primary care physician or endocrinologist to optimize their sugar control," Dr. Zobel says. "Uncontrolled insulin-dependent diabetes in the first trimester can lead to increased miscarriage rates and also a markedly increased risk of major birth defects."

Physical complications

A less common cause of miscarriage can be physical problems with the pregnant person, reports Dr. Nowacki, adding that this usually occurs in the second or third trimester. Here are some examples:

• Uterine fibroids can interfere with implantation or blood supply to the fetus.
• Some people are born with a uterine septum (also known as a septate uterus), an uncommon condition linked to miscarriage.
• Additionally, some may develop bands of scar tissue in the uterus from surgical procedures; this scar tissue can keep an egg from implanting properly and may hamper blood flow to the placenta.

A doctor can determine uterine defects through specialized X-rays before pregnancy. Most cases can be treated, which may reduce the risk of miscarriage.

Blood clotting disorders

Like physical abnormalities, miscarriages from blood clotting disorders (such as Factor V Leiden), are more rare, but they do occur. "I work a lot of people up on blood clotting disorders," explains Dr. Nowacki, "But they're just not as common as the other reasons."

Hormonal imbalances

Hormonal imbalances can also contribute to miscarriage. For instance, sometimes, the body doesn't produce enough of the hormone progesterone, which is necessary to help maintain the uterine lining to support the fetus and help the placenta take hold.

"Because this is not very common, we usually wouldn't test for it unless someone has had multiple miscarriages," says Jonathan Schaffir, M.D., an assistant professor of obstetrics and gynecology at Ohio State University College of Medicine. If hormonal imbalance contributed to miscarriage, medication may improve the odds of a successful subsequent pregnancy.

Drug, alcohol, or tobacco use during pregnancy

Some lifestyle habits—such as drug use, alcohol use during pregnancy, and smoking—have been found to cause early miscarriage and pregnancy loss in later trimesters. Optimizing your health leading up to your pregnancy and avoiding recreational drugs, tobacco products, and alcohol during pregnancy can help reduce your risk of miscarriage.

Immunological disorders and chronic illness

The ACOG believes that certain autoimmune disorders may play a role in miscarriage, especially in the case of recurrent miscarriage. Although the exact role of immunologic factors in miscarriage is complicated, according to Dr. Nowacki, the simplest way to understand it is that "the body just doesn't accept the pregnancy." Some research has found that certain antibodies may be among the most common causes of recurrent miscarriages.

"Lupus is an autoimmune disease that can result in an increased miscarriage rate, often due to anti-phospholipid antibodies that these women often carry," Dr. Zobel says. "Up to 5% of women can also carry these antibodies. Any woman who has had a recurrent miscarriage (greater than three spontaneous miscarriages), unexplained fetal death after 10 weeks, or a preterm birth before 34 weeks is recommended to be tested for anti-phospholipid syndrome. You cannot control whether you have these antibodies. However, if they are present, there are treatments available to reduce the risk of miscarriage and pregnancy loss."

Other chronic illnesses that may be linked to recurrent miscarriages include heart disease, kidney disease, and liver disease. If you have a chronic illness, find an obstetrician experienced in caring for people with your condition.

Problems with the placenta

Although more common with second-trimester miscarriages, problems with the placenta, including an abruption (when part or all of the placenta tears away from the uterine wall, leading to a loss of oxygen and nutrients to the embryo or fetus) or issues with the placenta itself, have been found in people who miscarried, according to a 2019 study.

Miscarriage Risk Factors

Other risk factors that may increase your risk of miscarriage include:

• Listeria: A bacteria that may be present in undercooked meats, raw eggs, and unpasteurized dairy products, listeria infections (listeriosis) during pregnancy may cause miscarriage.
• Maternal trauma: Trauma such as a car accident during pregnancy can cause miscarriage.
• Certain medications: Some medications, such as NSAIDs like ibuprofen, appear to inhibit the pregnancy hormone prostaglandin, which is needed for the embryo to implant properly.
• Advanced maternal age: Pregnant people over the age of 35 may have a higher risk of miscarriage.
• Infections: Infections such as Lyme disease can infect a growing fetus and lead to pregnancy loss, or Fifth disease, which can cause severe anemia in a growing fetus, causing miscarriage.
• Air pollution: Research shows that increased short-term exposure to nitrogen dioxide correlated with a higher risk of miscarriage.
• High fever: A high fever (above 102 degrees) during early pregnancy is most damaging to the embryo before 6 weeks.
• Polycystic ovary syndrome (PCOS): People with PCOS have a miscarriage risk factor three times higher than people who don't have PCOS, according to the National Institutes of Health. While researchers are still trying to understand fully why this risk is elevated, one theory is that it may be related to insulin resistance, which appears to be a common hallmark of PCOS.

What Does Not Cause Miscarriage?

There is plenty of misinformation circulating out there about what does and doesn't miscarriage. If you are worried about miscarriage, it is important to understand what the real risk factors are. According to ACOG, the following things do not cause miscarriage despite common misconceptions:

• Having sex
• Exercise
• Feeling stress
• Most incidents of falls or blows to the abdomen
• Arguments or sudden fright
• Working (unless you're working in conditions that are known to be dangerous such as with toxic chemicals)
• Spicy food
• Traveling
• Morning sickness or nausea
• Having taken birth control pills
• Most medications
• Being vaccinated

How Do I Decrease My Risk of Miscarriage?

In many cases, miscarriage can't be prevented, especially when the cause is at the chromosomal level. Even so, doctors advise optimizing your health before you conceive to give your pregnancy the best fighting chance—and that goes for both partners. You have the best chance of a healthy pregnancy when both partners contributing genetic material are healthy.

"Generally, I advise that people considering pregnancy see their OB-GYN to review chronic conditions and medications, begin prenatal vitamins two to three months prior to trying to conceive, ensure that all their vaccines are up-to-date, review their diet, and ensure they limit or eliminate alcohol and caffeine in their diets," recommends Dr. Zobel. "Those who use recreational drugs are advised to quit."

Keep in mind, though, that even if you followed all of that advice, you might not be able to prevent miscarriage from happening to you. Sometimes, miscarriages just happen. If you do experience a loss, remember that you are not alone. Don't hesitate to reach out to a health care provider and other professionals for resources and support, especially in trying to conceive again should you choose. Miscarriage may be common, but that doesn't mean it's not significant, and you deserve support along the way.