gynecology and obstetrics
Noninvasive Prenatal Fetal Screening Tests
Sequential First and Second Trimester Screening
Noninvasive first-trimester and second-trimester quad screening can be combined sequentially, with invasive fetal genetic testing withheld until results of second-trimester screening are available—whether first-trimester test results are abnormal or not. Sequential screening followed by amniocentesis for high-risk patterns increases sensitivity for Down syndrome to 95%, with a false-positive rate of only 5%. A variation of sequential screening, called contingent sequential screening, is based on the level of risk indicated by first-trimester screening: High risk: Invasive testing is offered without doing second-trimester screening. Intermediate risk: Second-trimester screening is offered. Low risk (eg,<1 in 1500): Second-trimester screening for Down syndrome is not offered because the first-trimester risk is so low. Patients with abnormal first-trimester, second-trimester, or sequential screening should be offered diagnostic testing (eg, amniocentesis). However, some patients may choose to pursue further testing for fetal trisomy with cell-free DNA (cfDNA) analysis ( Sequential first- and second trimester screening references ). Results of cfDNA testing may indicate low risk and be reassuring but are not definitive. Also, cfDNA testing may be inordinately expensive, and awaiting results of cfDNA testing delays definitive testing such as chorionic villus sampling or amniocentesis ( 1.American College of Obstetricians and Gynecologists/Committee on Genetics, and the Society for Maternal-Fetal Medicine: Practice bulletin no. 163: Screening for fetal aneuploidy. Committee on Practice Bulletins—Obstetrics,Obstet Gynecol127 (5):e123–e137, 2016. doi: 10.1097/AOG.0000000000001406 2.Norton ME, Jacobsson B, Swamy GK, et al:Cell-free DNA analysis for noninvasive examination of trisomy.N Engl J Med372 (17):1589-1597, 2015. doi:10.1056/NEJMoa1407349 