There are countless reasons for miscarriage, and many of them are beyond your control. Get insight into what causes miscarriage and learn who’s at risk for pregnancy loss.

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The first question that many couples ask after experiencing miscarriage is simple: Why? A miscarriage can be an incredibly devastating event with long-lasting emotional repercussions. But one of the most difficult aspects of a pregnancy loss is the complete lack of knowledge surrounding the reasons it happened.

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 amazing how often pregnancy actually goes 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 couples who suffer a miscarriage blame themselves, the truth is that they've probably done nothing to cause it. Here’s everything you need to know about the reasons for pregnancy loss, with tips for reducing your risk. 

Why Do Miscarriages Happen? 

According to the American Pregnancy Association (APA), the most common cause of miscarriage is a genetic abnormality in the embryo. But several other factors can also be the culprit, including thyroid disorders, diabetes, immunological disorders, drug abuse, and more. 

Chromosomal Abnormalities

Up to 70 percent of first trimester miscarriages and 20 percent of second trimester miscarriages occur because of a glitch in the fetus's genes, according to the March of Dimes. 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. "Genetically, (development) just stops," in these cases, explains Dr. Nowacki.

Miscarriages caused by chromosomal abnormalities happen more often in women 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. The frequency of miscarriage in women below age 20 is around 12 to 15 percent 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 (too low) or hyperthyroidism (too high), thyroid disorders can lead to problems with infertility or cause recurrent miscarriages. The Malpani Infertility Clinic's website explains that in cases where a woman's thyroid function is low, her body will try to compensate by producing hormones that can actually suppress ovulation. Conversely, a thyroid that is producing 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.


"Women 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 mother, 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 women are born with a septum, an uncommon uterine defect linked to miscarriage. 
  • Women may develop bands of scar tissue in the uterus from surgery or second-term abortions; 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

Sometimes a woman's body doesn't produce enough of the hormone progesterone, which is necessary to help 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 a woman's had multiple miscarriages," says Jonathan Schaffir, M.D., an assistant professor of obstetrics and gynecology at Ohio State University College of Medicine. Medication may improve the odds of a successful subsequent pregnancy.

Using Drugs, Alcohol or Tobacco During Pregnancy

Some lifestyle habits—such as drug abuse, 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 could help reduce your risk of miscarriage. 

Immunological Disorders and Chronic Illness

The American College of Obstetricians and Gynecologists thinks that certain autoimmune disorders play a role in miscarriage, especially with recurrent miscarriages. Although the exact role of immunologic factors in miscarriage is "complicated," according to Dr. Nowacki, she explains that the simplest way to understand it is that "the body just doesn't accept the pregnancy."

Some research has found that certain antibodies present could 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 percent 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 women with your condition.

Miscarriage Risk Factors

Other 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
  • Maternal trauma, such as a car accident
  • Certain medication
  • Advanced maternal age (over 35)
  • Infections such as Lyme disease or Fifth disease
  • Air pollution—A February 2019 study published in Fertility and Sterility found that increased short-term exposure to nitrogen dioxide correlated with a higher risk of miscarriage.
  • High fever (above 102 degrees) during early pregnancy—A high core body temperature is most damaging to the embryo before 6 weeks.

How Do I Decrease My Risk of Miscarriage?

In many cases, miscarriage can’t be prevented, especially when the cause is chromosomal abnormalities that aren’t compatible with life. Even so, doctors advise optimizing your health before you conceive to give your pregnancy the best fighting chance. 

"Generally, I advise that women considering pregnancy see their OB-GYN to review chronic conditions and medications, begin prenatal vitamins 2 to 3 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 may not be able to prevent miscarriage from happening to you.