This is a blood test that is offered to pregnant women who missed their combined screening test( performed between 11-14 weeks) to find out their chance of having a baby with Down syndrome. The test involves checking the blood levels of alpha feto protein(AFP), total human chorionic gonadotropin (hCG), unconjugated estriol(UE3), and Inhibin-A to calculate the risk for Down syndrome. In pregnancies with Down syndrome, the levels of AFP and UE3 are low, while hCG and Inhibin tend to be high Result of less than 1 in 250 is considered as screen positive in which case, diagnostic invasive testing like amniocentesis is recommended to confirm the diagnosis.

This test is usually performed between 16 to 20 weeks of gestation. It is important to be aware that this test is validated only in singleton pregnancies and therefore not offered to multiple pregnancies. In addition, if the results show higher values of AFP (2 to 3 times over the normal range), there is an increased risk of association with open spina bifida which would need a detailed fetal anomaly assessment by ulltrasound.