Prenatal Screening for Fetal Aneuploidy

Abstract

Prenatal genetic aneuploidy screening approaches are designed to identify pregnant patients at increased risk of having a fetus affected. Conventional prenatal screening has consisted in providing women a risk estimate of having a pregnancy affected with trisomy 21 or trisomy 18 based on maternal age and analysis of serum markers and ultrasound nuchal translucency (NT) measurement. In 2011, the introduction of cell-free DNA (cfDNA) based screening into clinical practice has provided new options for aneuploidy screening programs. Different protocols are currently in use, some that perform screening in the first trimester (combined first trimester screening), some in the second trimester (QUAD), some that integrate first and second trimester (serum integrated screening, integrated screening, sequential, or contingent screening). cfDNA screening can be implemented as a second tier / contingent screen or as a first tier screen and has the ability to detect chromosomal anomalies other than the common aneuploidies screened for by conventional screening. The choice of protocol will be based on local expertise and resources. Aneuploidy screening should also be offered to pregnant women with twin gestation although fewer validation studies in twins of the test being offered are available compared to studies done in singleton gestations. Professional societies and expert groups emphasize the need for pre-test and post-test counselling to ensure that women are making an informed decision. The general counselling points to be covered are similar regardless of the type of prenatal screening being offered. Prenatal screening programs should be implemented with resources that support patient and health care provider education, quality assurance of laboratory and NT services, access to genetic counselling services and diagnostic testing. The rapid pace of new developments in cfDNA screening for chromosomal anomalies brings new opportunities to enhance the performance of the screening but at the same time, challenges as the test menus are expanded and data is lacking as to the cost-effectiveness and clinical utility of implementing expanded panels.