Q&A: What's the risk of birth defects for older women?

First of all, congratulations on your pregnancy! The biggest problem for women in their 40s is actually getting pregnant—and you are already there. Remember that for women of all ages, taking 400 micrograms a day of folic acid can help avert some preventable birth defects such as spina bifida.

Still, we all have heard that older moms have increased chances of some genetic conditions in their offspring, and you are asking a good question about the magnitude of the possibilities in your situation.

Here are some numbers: If you look at age alone, the chance you are carrying a fetus with Down syndrome is a little more than 1 percent. At 35 years old, the chance is about half of 1 percent, and at younger ages, even less. By 45, the incidence of Down syndrome (DS) is about 2 percent. The risks for other genetic conditions also increase with age, but Down syndrome is the most prevalent.

Tests are available to identify genetic conditions like DS during pregnancy. Some tests are considered diagnostic tests, because they make the diagnosis, like chorionic villus sampling (CVS) and amniocentesis. Other tests are called screening tests. Screening tests assess your odds to help you make a decision about whether you want a diagnostic test. Screening tests include the first check (a combination of ultrasound and some bloodwork done at 11 to 14 weeks gestation) and the quad screen (a set of blood tests), which can't be done until about 16 weeks. Screening tests can tell you whether your chances of carrying a fetus with DS is one in 10, one in 100, or a one in 1,000. Decisions about diagnostic tests can be made based on the results of the screening tests.

It can be difficult to decide among the different testing options. I have some patients who say they would not terminate a pregnancy under any circumstances and they don’t want any testing that would just worry them, cost money, and (like CVS and amnio) pose a small risk to the pregnancy. Some moms who wouldn't terminate, though, still want to know so they can be prepared. Women who would terminate a pregnancy affected with DS need to get more information so they can make their decisions. Some choose to go directly to a diagnostic test, while others use the screening tests first to reassess the odds. Only a diagnostic test will give you a 100 percent accurate answer.

A genetic counselor is the best person to consult to help make these complex decisions. Genetic counselors will have more time than your doctor or midwife, and are specially-trained to help you clarify your personal values so you can make decisions that fit with your beliefs. Insurance will typically pay for genetic counseling for women who will be 35 by the due date.

Keep in mind as you go through this process that, based on your age, the odds are strongly in your favor that this baby will not have DS or a major birth defect. And don't forget your folic acid!