There are countless reasons for miscarriage to occur—and many of them are beyond your control. Get insight into the most common causes of miscarriage.

By Chaunie Marie Brusie, RN, BSN
moodboard Photography/ Veer

The first question that many women and couples want answered after experiencing a miscarriage is simple. Why?

A miscarriage can be an incredibly devastating event, with long-lasting emotional repercussions like depression, anger, fear, and guilt. But one of the most difficult aspects of a pregnancy loss is simply the complete lack of knowledge surrounding the reasons it happened.

Finding the Reasons for Miscarriage

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 it 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 miscarriage blame themselves, the truth is that there are few causes of miscarriage that are entirely preventable—though 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 Stephanie Zobel, M.D., an Ob-Gyn with Winnie Palmer Hospital. "ˆ or use recreational drugs are advised to quit." But even if you followed all that to the letter, you may not be able to prevent miscarriage from happening to you.

Common Causes of Miscarriage

1. Chromosomal Abnormalities

Hands down, the single most common reason that a miscarriage occurs is some a problem with either the egg or sperm's chromosomes during embryo formation. While some chromosomal abnormalities are compatible with life, (such as trisomy 21, the most common type of Down syndrome), other chromosomal disorders are simply incompatible with life. "Genetically, [development] just stops," explains Dr. Nowacki.

Miscarriages caused by chromosomal abnormalities have been found to happen more often in women older than age 35. "This is because the all the eggs that a woman will ever have she has from birth and the eggs age with her," Dr. Zobel says. "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."

2. Thyroid Disorders

Whether it be hypo (too low) or hyper (too high) thyroidism, 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 make the uterus unfavorable for implantation or lead to abnormal uterine bleeding.

3. Diabetes

"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."

If you have diabetes, the best plan of action is to see your doctor prior to becoming pregnant to optimize your health. "Chronic medical disorders including diabetes, hypothyroidism, hypertension, and autoimmune illnesses need to be addressed and well-controlled prior to pregnancy." Dr. Zobel says. "Establishing care with a physician early in the pregnancy is key to a successful pregnancy in women with chronic medical conditions."

4. Lifestyle

Some lifestyle habits – such as drug abuse, alcohol use during pregnancy, and smoking – have been found cause early miscarriage and pregnancy loss in later trimesters. Optimizing your health leading up to your pregnancy could help reduce your risk of miscarriage. "Up to half of pregnancies are unplanned, which means women are often not best prepared for pregnancy when it occurs," Dr. Zobel says. "Most women do not realize that they are pregnant until a couple weeks after their missed period. By that time, the fetal spinal cord has already been formed and the heart is beating. Preparing for pregnancy by modifying diet and exercise, limiting stress, optimizing chronic medical disorders, and beginning prenatal vitamins is ideal for all pregnancies. However, even this can not minimize miscarriage risk due to chromosomal abnormalities."

Another lifestyle factor that may increase miscarriage risk: living in a city. A February 2019 study published in Fertility and Sterility found that increased exposure to nitrogen dioxide is associated with a higher risk of miscarriage. Specifically, a 10-ppb increase in exposure over a seven-day span was linked to a 16% higher chance of miscarrying.

5. Physical Complications

Less common, but still significant occurrences of miscarriage can be caused by physical problems with the mother, such as uterine abnormalities including septum or polyps, or cervical incompetence, reports Dr. Nowacki. However, she states that in many instances, losses as a result of a physical complication will occur in the second or third trimesters.

6. Blood Clotting Disorders

Like physical abnormalities, miscarriages that result 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."

7. Immunological Disorders

Immunological disorders are a widely debated topic among women's healthcare providers, primarily because there is still much to be discovered about their role in contributing to miscarriage. However, the American College of Obstetricians and Gynecologists agrees that certain autoimmune disorders do 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."



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