All pregnant women have a risk of birth defects, including Down syndrome and other chromosome abnormalities. Most women don't know what their risk is, however, and may be surprised to learn what it is. The overall risk for all maternal ages in the U.S. is approximately 1 in 500 pregnancies at the time of a 2nd trimester ultrasound, based on the maternal age distribution. This risk has increased from 1 in 700 two decades ago, reflecting the trend toward delayed child birth.. Of course, the individual risk varies with the individual patient's age. The risk also decreases with gestational age because there is a certain 'attrition rate' due to the increased risk of spontaneous intrauterine death among fetuses with major chromosome abnormalities.
The table below summarizes the risk of fetal Down syndrome based on both maternal age and gestational age. The risk is reported as 1 in __. For example, a 30 year old woman has an approximate risk of 1:703 at 16 weeks.