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When I found out I was pregnant, I was simultaneously thrilled about having a baby and petrified that something would go wrong. Chalk it up to the occupational hazards of being a health writer, too much time spent trolling the Internet or a somewhat obsessive, glass-half-full-of-water-that-might-contain-toxins personality. Whatever the reason, I had a long list of bad things in my head. And miscarriage topped the list.
So I fretted. I worried. I ate spinach by the boatload and downed fistfuls of folic acid, all the while trying not to fall in love with the little bundle of cells dividing within me--because sometimes, I knew, they just don't make it. But mine did. And I've got a strapping 9-year-old son to prove it.
If you're like many pregnant women, the prospect of miscarriage looms high on the what-if list, as it did for me. But you can take comfort in the fact that while a certain number of pregnancies do end early, the vast majority result in healthy babies. Here's a look at why miscarriages happen, along with reassuring news about why your chances of having one might be lower than you think.
When chromosomes are the problem
The four main causes of miscarriage include problems with the embryo or fetus, and anatomical, hormonal and blood-clotting abnormalities within the mother. More than 50 percent result from the first--what experts refer to as "chromosomal errors."
"When the chromosomes of the egg and those of the sperm fuse to form an embryo, they usually pair up correctly," explains Henry Lerner, M.D., an OB-GYN at Newton-Wellesley Hospital in Newton, Mass., and a clinical instructor in obstetrics and gynecology at Harvard Medical School in Boston. "But sometimes they get scrambled. Since chromosomes are the blueprint for development, if they're paired incorrectly, the embryo stops developing at some point and dies."
These errors can be so significant that pregnancy per se never occurs--the fertilized egg may begin to divide and grow but fails to implant itself in the uterus. In fact, it's estimated that as many as half of clinical pregnancies--those in which fertilization does take place--never make it.
"The great majority of genetically abnormal embryos don't implant, and of the few that do, most miscarry early on," says William P. Hummel, M.D., a reproductive endocrinologist specializing in miscarriage and infertility at San Diego Fertility Center. "The body recognizes a problem and takes care of it before a woman even knows she's pregnant." From there, most miscarriages will happen within the first 12 weeks. "Once you pass the eight- to 12-week mark and we see the fetus's heartbeat by ultrasound, you have a 98 percent chance of having a full-term, healthy baby," Hummel adds.
Problems within the mother
Abnormalities in a woman's body--most often, her uterus--are the next most common cause of early losses, according to Hummel. "At least 30 percent of all miscarriages are due to anatomical factors within the uterus," he explains. "Of these, uterine fibroids are the biggest problem."
Hummel points out that although fibroids-noncancerous tumors are extremely common in women of reproductive age, not all result in miscarriages. "The key to a fibroid causing a miscarriage is its location and size," he says. The closer it is to the center of the uterus, where a fertilized egg is likely to implant, the more likely it is to cause problems; as for size, any fibroid greater than 5 centimeters (about 2 inches) can be problematic. Surgery to remove the fibroid is often extremely effective in reducing a woman's risk of miscarriage, Hummel adds.
The third most likely cause is hormonal. "Ten percent of miscarriages are related to hormonal events, particularly low progesterone levels," Hummel says. "Progesterone advises a woman's body to accept a foreign embryo as something desirable: It's nature's immunosuppressant." If a woman has low levels of this hormone, using progesterone supplements until eight to 10 weeks' gestation, when the placenta takes over the job of manufacturing it, dramatically increases the chances of a healthy outcome.
The fourth significant contributor to miscarriage is a relatively new discovery called thrombophilias, or blood-clotting disorders within the mother. "For women with thrombophilias, blood clots form in the smallest, most fragile blood vessels: those in the fetus, in the placenta and in the umbilical cord," Hummel explains. "These clots can impede blood flow and cause the fetus to die." Anti-coagulant treatments such as low-dose aspirin and heparin administered throughout pregnancy help prevent clotting and reduce the risk of miscarriage.