Which chromosomal disorders are primarily screened through pregnancy-associated plasma protein A levels?

The screening for chromosomal disorders during pregnancy often involves the measurement of specific biomarker levels, including pregnancy-associated plasma protein A (PAPP-A). PAPP-A is utilized primarily in the first trimester screening for Down syndrome (Trisomy 21) and Trisomy 18.

In the context of this question, Trisomy 21 is the most common chromosomal abnormality associated with significant clinical manifestation and is the focus of many screening protocols. Trisomy 18, also known as Edwards syndrome, can be detected through low levels of PAPP-A in combination with other markers.

While Trisomy 13 (Patau syndrome), Trisomy 16, 17, and other chromosomal abnormalities may have some associations in various screenings, they are not routinely screened specifically using PAPP-A levels. Thus, the association of PAPP-A levels with the likelihood of Trisomy 18 and Trisomy 21 makes this the correct answer.