Most commonly used sonological indicator for aneuploidy in first trimester is:

The most commonly used first-trimester screening protocol combines these two serum markers with the sonographic nuchal translucency. It can be achieved at 11 to 14 weeks. Also know: Two maternal serum analytes are tested in first trimester screening, hCG (or free bhCG) and pregnancy-associated plasma protein A (PAPP-A). In the first trimester, serum hCG levels are higher, approximately 2.0 MoM, and PAPP-A levels are lower, approximately 0.4 MoM, with a Down syndrome fetus. Ref: (2010). Chapter 13. Prenatal Diagnosis and Fetal Therapy. In Cunningham F, Leveno K.J., Bloom S.L., Hauth J.C., Rouse D.J., Spong C.Y. (Eds), Williams Obstetrics, 23e.