<prompt> Most commonly used sonological indicator for aneuploidy in first trimester is:Gestational sac volumeCrown-rump lengthNuchal translucencySerum P HCG level  <response>: 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. 