Sayres, L. C., & Cho, M. K. (2011). Cell-free fetal nucleic acid testing: A review of the technology and its applications. Obstetrical & Gynecological Survey, 66, 431–442. Norton, M. E., & Wapner, R. J. (2015). Cell-free DNA analysis for noninvasive examination of trisomy. The New England Journal of Medicine, 372, 1589–1597.