FRIDAY, April 6 (HealthDay News) -- Women who'd like to become pregnant -- especially those who are taking medications for chronic conditions -- may need to add something to their to-do list: Plan, plan, plan.

That's because some medications are known to cause birth defects. Avoiding all medications during pregnancy is not always possible, however, and sometimes not taking a prescribed medication could be harmful, too.

"The best thing for women to do, all women, is to plan their pregnancy," said Dr. Diane Ashton, deputy medical director for the March of Dimes. "Women should speak with their physician about their plans to become pregnant. If they have a condition that requires medication, that's an ideal time to switch to medications that have less risk, if necessary. And, it gives time for the condition to be stabilized."

About 120,000 babies born each year in the United States -- or about one of every 33 -- are born with a birth defect, something abnormal that occurs in a baby's development, according to the U.S. Centers for Disease Control and Prevention. Some are more cosmetic than dangerous, such as an extra toe, but others are serious and include heart defects and spina bifida. Birth defects are the leading cause of death in babies younger than 1 year, according to the National Birth Defects Prevention Network.

Most birth defects happen in the early stages of pregnancy, often before a woman even knows she's pregnant, according to the CDC.

That's why it's important for women who are using potentially dangerous medications -- such as retinoic acid treatments for acne -- to plan a pregnancy and, Ashton said, to use contraception while they're on those drugs.

Dr. Deborah Campbell, director of neonatology at Montefiore Medical Center in New York City, concurred.

"The most dangerous time is the first trimester, especially between weeks three and eight, because that's when all of the organs are forming," Campbell said. "And, half of all pregnancies in the U.S. are still unplanned so women may not even know they're pregnant at this crucial time. We can't overemphasize enough the importance of planning your pregnancy."

Women with diabetes, women at risk for diabetes and women who have high blood pressure, seizure disorders, depression or autoimmune diseases who are on medications need to review these medications with their obstetricians, Campbell said.

"Where at all possible, switch to medications that are the least likely to cause birth defects," she suggested. "If medications can't be stopped, the pregnancy can be more closely monitored. What's important is that the mother is in the best shape possible. For example, you can't just stop seizure medications. A woman who's having seizures poses a risk to her fetus. It's important to weigh the relative risks and benefits with your obstetric care provider."

What are the risks? As Ashton explained:

• Drugs that affect women's hormones can cause their babies to be born with problems with their genitalia.
• Certain anti-seizure medications can cause heart defects, spina bifida and cleft lip.
• Retinoic acid has been associated with a host of birth defects, including malformations of the face, heart and brain.
• Some antidepressants have been linked to heart defects.
• Cancer-fighting drugs, which target rapidly dividing cells, pose a significant risk for miscarriage as well as the potential for central nervous system and brain defects.

But, as scary as all that sounds, Ashton said, it's important to remember that even with exposure to drugs known to cause birth defects, most women will have a normal baby.

Still, medical experts agree that it's best to avoid medication exposure as much as possible during pregnancy.

"Anything you're going to take during pregnancy, you should check with your doctor," advised Campbell.

The warning applies to more than prescription drugs, too -- including all over-the-counter medications as well as herbs and dietary supplements.

"People assume because you can buy something over-the-counter that it's safe and you don't have to worry about it," Campbell said. "But there are some herbal preparations that can be a risk for infants, and there may be adulterants in the product. These may not cause birth defects, per se, but could cause neurologic changes that could cause irritability and behavior problems."

More information

The Nemours Foundation has more on birth defects.

For more on preventing birth defects, read about one woman's efforts to manage medications and pregnancy.