Which of the following trisomies is NOT among those associated with decreased AFP?

• A. Trisomy 13 (Patau)
• B. Trisomy 18 (Edward)
• C. Trisomy 21 (Down)
• D. Trisomy 17

Answer: (D)

AFP levels in maternal serum are used in prenatal screening because they reflect fetal production. In the second-trimester screen, a pattern of decreased AFP is commonly seen with certain fetal aneuploidies, especially trisomy 21, trisomy 18, and trisomy 13, where the reduced fetal protein shows up alongside other marker changes. This reduction helps flag pregnancies at higher risk for these conditions and prompts further evaluation.

Trisomy 17 does not have a characteristic association with decreased AFP on standard screening. It is a rarer chromosomal abnormality and does not fit the typical AFP pattern used to screen for the common trisomies, so it’s not considered among those linked to low AFP. Therefore, the trisomy not associated with decreased AFP is trisomy 17.