What Does -- and Doesn't -- Cause Miscarriage

Miscarriage occurs in up to 15 percent of known pregnancies. Most of the time, there's not a lot you can do to prevent miscarriage (a few notable exceptions below*), but it helps to know why it happens.

• Chromosomal problems
  
  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. "It's still a mystery to us," says Jonathan Schaffir, MD, an assistant professor of obstetrics and gynecology at Ohio State University College of Medicine, "but sometimes the two sets of chromosomes from the egg and the sperm don't line up right at the moment of conception. It doesn't mean that anything's wrong with the mother or father -- or that it will necessarily happen the next time.
  
  The older you are, the more likely it is that your eggs may have faulty chromosomes. Although there's no one "red flag" age, most doctors will do more genetic testing in women over 35. "But 45-year-olds can still get pregnant and have healthy babies," says Dr. Schaffir.

• Chronic medical conditions
  
  Certain illnesses, especially those that restrict blood flow to the uterus, may increase a woman's chances of miscarrying (because the growing fetus can't get enough oxygen to survive). These include diabetes, thyroid disease, lupus, and heart disease, as well as others like uterine infections. Managing the condition before and during pregnancy can reduce miscarriage risk.

• 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 Dr. Schaffir. Medication may improve the odds of a successful subsequent pregnancy.

• Excess caffeine intake
  
  Women who consumed 200 milligrams or more of caffeine each day (about two cups of regular coffee or five 12-ounce cans of caffeinated soda) had twice the miscarriage risk as those who didn't have any, according to a new study by Kaiser Permanente in the American Journal of Obstetrics and Gynecology.
  
  The cause isn't known for sure, but researchers suspect that the chemical crosses the placenta and can affect developing cells. While one cup of java or soda a day is probably fine, consider being extra-safe and switching to decaf entirely while you're actively trying to conceive and during pregnancy. "When so many other risk factors are out of your control, this is one of the few things you can do to improve your baby's health," says Tracy Flanagan, MD, director of Women's Health, Kaiser Permanente Northern California. "Why not err on the side of caution?" As always, if you have any concerns about what's safe for you, discuss them with your healthcare provider.

• Excess alcohol, cigarette, and drug use
  
  Had a glass of wine or two before you knew you were expecting? Don't worry -- that's not the level of use we're talking about here. "It's exposing a fetus to large amounts of these chemicals on a regular basis that can cause miscarriage, because they have a poisonous effect on all those developing cells," says Dr. Schaffir. But as soon as you suspect you're expecting or a test confirms it, cut out the cocktails, cigarettes, and such for good.