Which maternal serum marker is associated with a risk for T18?

The correct answer is associated with lower levels of Pregnancy-Associated Plasma Protein A (PAPP-A) during the first trimester of pregnancy. A significant decrease in PAPP-A has been linked to an increased risk of various fetal aneuploidies, including Trisomy 18 (T18). PAPP-A plays a role in the normal development of the placenta and fetal growth, and low levels can signify potential complications.

In contrast, other markers listed do not specifically correlate with Trisomy 18. High levels of alpha-fetoprotein (AFP) are often associated with neural tube defects rather than chromosomal abnormalities. Similarly, low estriol levels are more commonly associated with Down syndrome (Trisomy 21) rather than T18. High levels of human chorionic gonadotropin (hCG) can be elevated in several conditions as well, but they are particularly associated with Trisomy 21 rather than Trisomy 18. This highlights the specificity of PAPP-A in assessing the risk for T18.