What is the expected alpha-fetoprotein (a-FP) level in a quad screen test for a baby with Down syndrome?

In the context of a quad screen test for Down syndrome (trisomy 21), the expected level of alpha-fetoprotein (AFP) is typically low. This marker is one of the key components measured in the quad screen, which also includes human chorionic gonadotropin (hCG), estriol (uE3), and inhibin A.

When a fetus has Down syndrome, several changes occur in the levels of these substances. In particular, studies show that the AFP level tends to be lower in pregnancies affected by Down syndrome compared to normal pregnancies. This low level of AFP is attributed to the abnormal placentation and altered fetal metabolism commonly associated with this chromosomal anomaly.

In contrast, in cases of neural tube defects or abdominal wall defects, AFP levels are generally elevated, indicating that decreased AFP levels can be a significant marker for identifying pregnancies affected by Down syndrome. This understanding is crucial for interpreting the results of the quad screen effectively and providing appropriate counseling and follow-up for expectant parents.