Which trisomy is NOT typically associated with decreased AFP in maternal screening?

• A. Trisomy 13
• B. Trisomy 18
• C. Trisomy 21
• D. Trisomy 20

Answer: (D)

A key idea here is how maternal serum AFP levels shift in fetal trisomies used in screening. AFP is produced by the fetus, and in pregnancies affected by many trisomies, the AFP level in the mother’s blood is often lower than normal. This decrease helps screen for certain chromosomal abnormalities, with trisomies such as 21 (Down syndrome) and 18 (Edwards syndrome) showing a tendency toward reduced AFP on screening. In contrast, trisomy 20 does not have a characteristic pattern of decreased AFP, so it isn’t typically associated with lower AFP levels in maternal screening. Therefore, trisomy 20 is the one that does not fit the pattern.