There's no bigger shock than going from the thrill of a pregnancy to the letdown of a miscarriage. Yet one in five pregnancies ends this way; 60 percent of these are due to genetic abnormalities (often a tripling of a chromosome) that can't be avoided. Although the cause of miscarriage is usually out of a woman's control, you can increase your odds of having a successful pregnancy by taking charge of your health, says Henry Lerner, M.D., a Massachusetts-based ob-gyn and author of Miscarriage: Why It Happens and How Best to Reduce Your Risks. Here's his advice:

Get screened. Ask your doctor to test you for sexually transmitted diseases (STDs). Untreated, STDs such as gonorrhea, syphilis, HIV, and herpes increase the risk of miscarriage and can harm your baby.

Exercise in moderation. While it's fine to work out every day, the triathlon may have to wait. Doctors speculate that excessive physical activity (such as heavy running) may cause miscarriage because it elevates body temperature and can reduce blood flow to the fetus. To play it safe, exercise in moderation and avoid activities (such as skiing or horseback riding) that could cause you to lose your balance or lead to an abdominal-area injury.

Check your vaccination record. Certain diseases can increase your risk of miscarriage. If you suspect you may have missed routine immunizations as a child, talk to your doctor  -- she can do a blood test to see whether you're immune. The best time to get immunized is before you try to become pregnant.

Watch what you eat. Two rare infections, listeria and toxoplasmosis, can up your risk. Avoid unpasteurized dairy products (soft cheeses like brie and camembert) and undercooked or raw meat.

Talk to your doctor. Chronic conditions like thyroid disease, epilepsy, and lupus can increase your risk of miscarriage  -- though most women with these illnesses can have healthy babies. Also tell your doctor about any family history of infertility, clotting disorders, recurrent miscarriages, or other disease.

Avoid tobacco, alcohol, and illegal drugs. Although this would seem like a no-brainer, many women don't realize that in addition to being harmful to the fetus, these substances can cause miscarriage. Watch your caffeine intake. Studies have had mixed results, but Dr. Lerner suggests limiting caffeine during pregnancy to no more than 200 milligrams a day (about two cups of coffee).

Stay positive. About 75 percent of women who miscarry go on to have successful pregnancies. To learn more, visit SHARE Pregnancy & Infant Loss Support ( www.nationalshareoffice.org).

---