Term	Definition
Noninvasive prenatal testing	Amplification of the placental cell-free DNA circulating in the maternal bloodstream to determine the likelihood of fetal aneuploidy
First-trimester combined screening	Combination of nuchal translucency testing and maternal serum measurement of PAPP-A and free or total hCG levels
Second-trimester quadruple (quad) screening	Combination of alpha fetoprotein, unconjugated estriol, hCG, and inhibin A levels from maternal serum to produce a single risk estimate
Integrated screening	First-trimester nuchal translucency and PAPP-A testing are integrated with second-trimester quad screening to produce a single risk estimate; results are withheld until after second-trimester quad screening; serum integrated screening is an alternative method that omits first-trimester nuchal translucency testing
Stepwise sequential screening	First-trimester combined screening (nuchal translucency, PAPP-A, and hCG) is used to determine risk; patients at high risk are offered invasive diagnostic testing (chorionic villus sampling or amniocentesis), and patients at low risk receive second-trimester quad screening to refine the risk estimate
Contingent sequential screening	First-trimester combined screening (nuchal translucency, PAPP-A, and hCG) classifies patients as low, intermediate, or high risk; low-risk patients need no further testing, intermediate-risk patients may have second-trimester quad screening to refine the risk estimate, and high-risk patients are offered invasive diagnostic testing (chorionic villus sampling or amniocentesis)
Sensitivity (detection rate)	The percentage of individuals with a condition correctly identified as positive for that condition; depends on the characteristics of the test
Specificity	The percentage of individuals without a condition correctly identified as negative for that condition; depends on the characteristics of the test
Negative predictive value	The likelihood that a negative test result reflects a true negative (the condition is not present); depends on the test and the prevalence of the condition in the population screened
Positive predictive value	The likelihood that a positive test result reflects a true positive (the condition is present); depends on the test and the prevalence of the condition in the population screened