Alpha-fetoprotein (AFP) screening is a blood test done on the woman between the fifteenth and eighteenth weeks of pregnancy to screen for feta abnormalities. AFP is a protein produced by the baby. A high level of AFP in the woman’s blood indicates a possible open neural tube defect, such as spina bifida or anencephaly. A high level may also be caused by a multiple pregnancy or a miscalculation of gestational age. A low level may indicate Down syndrome. Also measuring two other substances – human chorionic gonadotropin and unconjugated estriol increases the accuracy of detecting Down syndrome, identifying about 70 percent of Down syndrome cases.
AFP testing has a history of false positive readings, however, and further testing should be done if the results are abnormal. The additional tests could include another AFP blood test, ultrasound for accurate determination of gestational age, or amniocentesis and for a more accurate diagnosis.