Q&A: I'm curious about my risk of having a child with Down syndrome.

You have valid concerns about bearing a child with Down syndrome. In general, for all women, the average likelihood of delivering a child with Down syndrome is about 1 in 800. At your age (30), the average risk is about 1 in 710. This risk can be defined more accurately when you have the routine blood test (called the maternal serum quadruple screen) usually done around 16 weeks of pregnancy.

About 94 percent of all children born with Down syndrome are characterized by having an extra chromosome number 21 (called trisomy 21 Down syndrome). About 3 percent or 4 percent of infants born with Down syndrome have a structural chromosome rearrangement involving chromosome number 21 (called a translocation type of Down syndrome). It is this type that is potentially hereditary, the structural abnormality being transmitted by one parent to a child. Because of the possibility that this translocation type exists in your family, every effort should be made to determine the precise chromosome abnormality your fiancé's uncle had.

If it proves impossible to find out that result or if he had a translocation, a blood sample from your fiancé should be obtained for chromosome analysis. The odds are obviously small that he will prove to be a carrier of the translocation. However, in the unlikely event he proves to be a carrier, there would be a 2 percent to 4 percent probability of having a child with the translocation type of Down syndrome. Early pregnancy studies via amniocentesis and chromosome analysis of fetal cells would enable timely prenatal diagnosis and almost certain reassurance that the fetus is healthy. Pregnancy termination is available in the event fetal Down syndrome is diagnosed.

Incidentally, while your risk of having a baby with birth defects increases with age (especially once you reach 35), your fiancé's age isn't a factor.