A 38 years old female, mother of a child with Down's syndrome became pregnant again. The method of assessing the risk of Down's syndrome in the present pregnancy is:

Chorionic villus sampling is generally performed at 11-12 weeks' gestation. Tissue obtained by chorionic villus sampling provides DNA for molecular analysis and contains dividing cells (cytotrophoblasts) that can be rapidly evaluated by FISH. The primary advantage of villous biopsy is that results are available earlier in pregnancy, which lessens parental anxiety when results are normal. It also allows earlier and safer methods of pregnancy termination when results are abnormal. Ref: Cunningham F.G., Leveno K.J., Bloom S.L., Hauth J.C., Rouse D.J., Spong C.Y. (2010). Chapter 13. Prenatal Diagnosis and Fetal Therapy. In F.G. Cunningham, K.J. Leveno, S.L. Bloom, J.C. Hauth, D.J. Rouse, C.Y. Spong (Eds), Williams Obstetrics, 23e.