My fiancee and I recently found out she is pregnant. She is 36 years old with a healthy two-year-old boy. On our first visit to the doctor, he asked us both if there were any birth defects in our families. On my side there is not, but she has two uncles with Down syndrome. I was shocked when the doctor told us there would be a 1 in 300 chance we could have a Down baby. While these are still odds in our favor, it had never occurred to me and now I am quite worried. Is his estimate within reason? Is it her age that presents the risk, or the fact it exists in her family? Does the fact that she has a healthy two-year-old lessen the chance, and does my family tree offer any help?

-- Gary

What your doctor was referring to was the age-related rate of chromosomal abnormalities; the fact that your fiancee has 2 relatives with Down syndrome was not taken into account. The absence of problems in your family does not matter. In a woman who will be 36 at the time of delivery, the risk of having a baby with Down syndrome is 1:275. By age 40, that risk increases to 1:100. This age-related increase in the risk of Down syndrome and other chromosomal abnormalities is the reason we offer genetic testing to all women who will be 35 or older at the time of delivery.

There are two tests available to determine whether the baby your fiancee is carrying is affected with Down syndrome or other chromosomal problems. One test, called chorionic villus sampling, may be done around 11 weeks of pregnancy and is performed by specialists with much experience. It involves threading a small catheter through the cervix into the area where the placenta (the organ that nourishes the baby during pregnancy) is forming. A small amount of this tissue is extracted and sent to the lab for analysis. The other test, amniocentesis, is usually done around 15 or 16 weeks of pregnancy and may be done by most obstetricians. In this test, a long skinny needle is inserted through mom's abdomen into the fluid surrounding the baby; this too is sent to the lab for analysis. Results from chorionic villus sampling are available more quickly than those from amniocentesis.

Neither test is without risks, including the possibility of miscarriage. The risk for a miscarriage is about 1 in 200-300 after an amniocentesis and about 1 in 100-200 after a chorionic villus sampling.

You and your fiancee should discuss these options and risks with your doctor. Only you can decide whether you want the tests done and what you would do if faced with a chromosomal abnormality. Remember, if the chance of having a problem is 1:275, the chance of everything being normal is 274:275! You should to discuss all of these issues with your doctor.