Apollo Medicine (Jan 2023)
Fetal medicine and current practice of prenatal screening
Abstract
Prenatal screening has come a long way from the discovery of fetal ultrasound markers for Down syndrome followed by improved ultrasound skills where genetic syndromes and major congenital malformations can be diagnosed right in the first trimester itself. The outcome is a key factor for the couple to make informed decision on the future of the high risk pregnancy. Prenatal screening,diagnosis,management and subsequent prevention is now an established practice in fetal medicine. The first trimester screening (at 11-13+6 weeks) includes evaluating markers for common aneuploidies (trisomies) such as Down syndrome along with a detailed fetal anatomy review. This is followed by a genetic sonogram or anomaly scan at around 20 weeks. Any signs of “high risk” for trisomies or malformations prompt invasive testing for precision diagnosis, either a chorion villous sampling (CVS) or an amniocentesis (AC). The fetal sample is used for various genetic tests, such as chromosome microarray analysis (CMA), targeted clinical exome sequencing (CES), or whole exome sequencing (WES) depending upon the abnormality noted or the suspected genetic condition. Non Invasive prenatal screening (NIPS) is now available as a screening tool for Down syndrome (99%accuracy for Down syndrome) if the couple choose to defer invasive testing. Apart from aneuploidies, prenatal screening for non-structural disorders, such as Beta thalassemia (both parents carriers) or investigating any other genetic disorder can also be offered. In addition, Carrier Screening can be considered especially by consanguineous couples who are known to be at increased risk for having an affected baby. The option for NIPS is now preferred by most parents undergoing pre-conception genetic counselling and confirmed to be carriers for an autosomal recessive genetic disease. The prenatal screening is the best choice for secondary prevention of genetic condition. The option for preimplantation genetic diagnosis (PGD) offers primary prevention by specific wild embryo selection and implantation in an in vitro fertilization cycle, following exclusion of affected embryo for aneuploidy (PGD-A), structural chromosome aberration (PGD-A), and a monogenic Mendelian genetic disease (PGD-M). The prenatal screening or diagnosis by the Fetal Medicine Team remains the mainstay for identification and management of pregnancies at high risk of genetic disorders.
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