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.