Scientists know more than ever about what causes it -- and which women may be at higher risk.

By Disney Princesses as Career Women
June 11, 2015

Who's at risk?

When a nine-week ultrasound revealed that she'd had a miscarriage, Lisa Pisha was desperate for answers. "I just wanted to know why I lost my baby," says Pisha, 30, of Naperville, Illinois. "Was it something I ate, or drank, or did during my pregnancy? Was it just a fluke?" But no one -- not her doctor or her friends, or even the miscarriage Web sites she studied -- could offer a definitive answer. "When I got pregnant again, I was on pins and needles for my entire first trimester," Pisha says. Fortunately, things went smoothly, and her daughter, Graham, was born last April.

Officially, about 15 percent of all known pregnancies end in a miscarriage, meaning they terminate before the 20th week. (After that, the loss is considered a stillbirth, according to the March of Dimes.) But experts say the actual number may be closer to 50 percent when it includes early miscarriages that happen before a woman even realizes she's pregnant. And virtually everyone, regardless of age, health, and previous successful pregnancies, is vulnerable. "No woman is immune," says Katherine E. Hartmann, MD, PhD, director of women's health research at Vanderbilt University Medical Center, in Nashville. Because miscarriages happen so often, most doctors don't even evaluate women until they've had at least two.

Still, women and physicians are understandably eager to learn more about what can trigger a miscarriage, and the latest research is uncovering some surprising possibilities.

Chromosomal Abnormalities

Usually, losing a pregnancy is a one-time tragedy; fewer than 2 percent of women miscarry more than once. In as many as 80 percent of cases, the cause is a random chromosomal abnormality in the fetus, says Charles J. Lockwood, MD, chair of obstetrics and gynecology at the Yale University School of Medicine. Moms over 35 are most at risk, since the older a woman gets, the more vulnerable her eggs are to defects.

Health Conditions

Beyond chromosomal damage in the fetus, certain health conditions in moms may make miscarriage more likely. For example, diabetic women can have excessive glucose in their bloodstream that causes them to miscarry, says Elena Yanushpolsky, MD, director of reproductive surgery at Brigham and Women's Hospital, in Boston. She instructs her diabetic patients to make sure they have healthy insulin levels for at least three months before they try to conceive.

Polycystic ovarian syndrome, another condition that raises insulin levels, can be a culprit. So can abnormal thyroid hormone levels. "Women prone to thyroid problems, especially those taking thyroid medications, should have their blood hormone levels tested regularly throughout pregnancy," says Samuel Refetoff, MD, director of the endocrinology lab at the University of Chicago. His study of women with genetically high thyroid levels found that the condition tripled their risk of losing a baby.

An autoimmune disorder called antiphospholipid syndrome, in which a woman's body makes antibodies that lead to blood clots in the placenta, is another cause of miscarriage, especially in the second trimester. (If a woman tests positive for these antibodies, her doctor may prescribe a blood thinner such as heparin or low-dose aspirin to reduce her miscarriage risk.) Still another risk factor: a malformed uterus, a condition that can usually be corrected by surgery.

Other suspected miscarriage culprits are still controversial. Some doctors think a woman can miscarry when she doesn't produce enough progesterone, which surges in the second half of the menstrual cycle and helps prepare the uterus to receive the fetus. "The problem is that progesterone spikes intermittently during a cycle, so getting a blood test isn't very useful," says Parents advisor Jamie Grifo, MD, PhD, director of reproductive endocrinology at the New York University School of Medicine.

Fertility experts are also uncertain if the immune system plays a role in attacking an embryo. "Natural killer cells have been found in women who miscarry," Dr. Grifo says. "But these cells have also been found in women who've just had a baby; we don't know what role they actually play."

Finally, researchers are reexamining some conventional theories. For instance, many are having second thoughts about the role of fibroids -- benign growths in the uterus. "We used to think that the biggest fibroids were the biggest troublemakers, but we're finding that many don't grow during pregnancy, and about a third of all fibroids actually shrink," says Dr. Hartmann. One type that does seem to increase miscarriage risk is the rare submucosal fibroid, which grows underneath the uterine lining.


Recent studies have debated whether drinking coffee increases the risk of pregnancy loss.

Although research at Kaiser Permanente found that consuming 200mg or more of caffeine daily may double a woman's risk of miscarriage, another study from the Mount Sinai School of Medicine, in New York City, and other institutions concluded that up to 350mg of caffeine doesn't make a difference. So which is right? A problem with both reports is that they rely on women's faulty memory of their eating and drinking habits over several months, explains Dr. Hartmann, a coauthor of the second study. The American College of Obstetricians and Gynecologists (ACOG) recently released new guidelines saying that there's no association between moderate intake of caffeine and miscarriage or preterm birth. According to ACOG, it's perfectly safe for pregnant women to have 200mg of caffeine per day -- equal to about one 12-ounce cup of coffee or about five cans of a soft drink that contains caffeine.

Pain Pills

Taking aspirin or ibuprofen may almost double the risk of early pregnancy loss, according to another Kaiser study. Both work by suppressing the body's prostaglandins, chemicals that help the embryo implant in the uterus, says study coauthor De-Kun Li, MD, PhD. "Women who took these pills around the time of conception or for a week or more afterward had the highest risk of miscarriage," he says. Since acetaminophen works in an entirely different way it was found not to cause problems.


Several years ago, a group of birth-defects specialists declared obesity a pregnancy risk factor, partly because obese women (those with a BMI over 30, which, for a 5'4" woman is 174 pounds or more) are more likely to miscarry. Experts aren't sure why there's a link, but they note that many obese women are also insulin resistant. "Women should be cautious about dieting during pregnancy, but they can certainly lose weight before conception and eat in a healthy way once they conceive," says Edward E. Wallach, MD, professor of reproductive endocrinology at the Johns Hopkins University School of Medicine, in Baltimore.


A study of nearly 1,500 Swedish women found that those with the lowest levels of folate (a nutrient that also helps prevent neural-tube defects) had a 50 percent higher chance of having a miscarriage. Researchers note this B vitamin is crucial for proper cell growth and embryo development. Women can easily reach the government recommendation of 400mcg a day by taking a prenatal vitamin or by eating folate-fortified breakfast cereals or other grains and leafy greens.


Several types of viral infection have been implicated in miscarriage, including varicella, which causes chickenpox; rubella, which causes German measles; and HIV, which causes AIDS. Certain bacteria may also be to blame: An Austrian study found that having bacterial vaginosis (BV) seemed to double the risk of a second-trimester miscarriage. (The researchers didn't examine first-trimester loss, which is far more common.) In the U.S., women who don't show BV symptoms usually aren't screened. But study coauthor Herbert Kiss, MD, says his hospital now routinely tests for common genital-tract infections late in the first trimester or early in the second and has seen miscarriage rates go down. Finally, listeriosis, caused by certain bacteria found in food, can be a risk. The FDA urges pregnant women to avoid hot dogs and luncheon meats that aren't steaming hot, as well as smoked seafoods like lox, and unpasteurized milk and soft cheeses.

Originally published in the October 2008 issue of Parents magazine.

All content on this Web site, including medical opinion and any other health-related information, is for informational purposes only and should not be considered to be a specific diagnosis or treatment plan for any individual situation. Use of this site and the information contained herein does not create a doctor-patient relationship. Always seek the direct advice of your own doctor in connection with any questions or issues you may have regarding your own health or the health of others.

Parents Magazine