Amniocentesis, often called amnio, is recommended for women who will be older than 35 on their due-date.
Thirty-five is the recommended age to begin amnio testing because that is the age at which the risk of carrying a fetus with such a defect roughly equals the risk of miscarriage caused by the procedure-about one in 200.
Because this test has a high false-positive rate, another test such as amnio is recommended whenever the AFP levels fall outside the normal range.
It is possible to perform an amnio as early as the 11th week, but this is not usually recommended because there appears to be an increased risk of miscarriage when done at this time.
The advantage of early amnio and speedy results lies in the extra time for decision making if a problem is detected.