Screening for Genetic Diseases
- Prenatal genetic testing gives information about whether the fetus has certain genetic disorders or not.
- Approximately 21,400 children with Down syndrome, 9000 with beta-thalassaemia and 5200 with sickle cell disease are born in India every year.
- Neural tube defect (NTD) is the most common congenital malformation.
Screening for Genetic Diseases
Some mothers are tested medically to rule out the genetic diseases like chromosomal defects or Down Syndrome. Screening Tests for Genetic Disease relate to identification of diseases that arise due to changes of chromosomes, genes, or proteins. The screening is essential to find out the possibility of developing a genetic disorder.
The screening is quite common amongst the pregnant mothers. Dr Mayuri Kothiwala strongly takes into consideration the genetic disorder tests that include:
- Prenatal Diagnosis
- Predictive/ Predisposition
- Preimplantation Genetic Diagnosis (PGD)
What is prenatal genetic testing and why is it necessary?
Prenatal genetic testing includes a battery of diagnostic tests that can detect genetic disorders, if any, in the unborn fetus. It thus gives parents-to-be information about whether their fetus has certain genetic disorders or not and if any life threatening disorder is suspected, a timely decision can be taken to terminate the pregnancy.
What are genetic disorders?
A disorder that is caused by changes in a person’s genes or chromosomes during the embryonic development is a genetic disorder. Genetic disorders can be due to a change in the total number of chromosomes (aneuploidy) or due to deletion of genes in a chromosome (mutations). Examples of aneuploidy are trisomy, where there is an extra chromosome and monosomy, when a chromosome is missing. Examples of mutation include sickle cell disease, cystic fibrosis, and many others.
What are the types of prenatal genetic tests?
There are two main types of prenatal tests for genetic disorders:
Prenatal screening tests:These tests can tell you the probability that your fetus has for developing an aneuploidy and a few other disorders. These include ultrasound, serum double marker/ quadruple marker test, cell-free DNA testing and many more.
Prenatal diagnostic tests:These tests are done in cases where the screening tests are positive for some specific disorder and can actually confirm whether the fetus thas certain disorders or not. These are invasive tests done on cells from the fetus or placenta obtained through amniocentesis or chorionic villus sampling (CVS).
What is first-trimester screening?
- SFirst-trimester screening includes a blood test of the pregnant woman (double marker test) and an ultrasound examination (NTNB scan). Both tests are done together between 10 weeks and 13 weeks and 6 days of pregnancy:
- The double marker testmeasures the level of two substances- beta HCG and PAPP-A.
- In the ultrasound examination,NT or nuchal translucency (thickness of a space at the back of the fetus’s neck) is measured along with NB or nasal bone. Any abnormal measurement is suggestive of an increased risk of aneuploidies.
What is second-trimester screening?
Second-trimester screening includes the following tests:
- The Quadruple markeror “quad test” is a blood test that measures the levels of four different substances in the blood- AFP, HCG, Estriol, Inhibin A. The quad test screens for Down syndrome, trisomy 18, and NTDs. Ideally, it is done between 15 and 18 weeks of pregnancy.
- An ultrasound exam done between 18 weeks and 20 weeks of pregnancy, also known as Level II anomaly scan, checks for major physical defects in the brain and spine, facial features, abdomen, heart, and limbs.
What is cell-free DNA testing?
Cell-free DNA is the small amount of DNA that is released from the placenta into a pregnant woman’s bloodstream. This can be used in screening pregnant women for Down syndrome, trisomy 13, trisomy 18 and problems with the number of sex chromosomes. It can be done starting at 10 weeks of pregnancy and positive warrants a diagnostic test with amniocentesis or CVS.
What do the different results of prenatal screening tests mean?
- A positive screen test result for aneuploidy means that the fetus is at high risk of having the disorder as compared to the general population. It does not mean that your fetus definitely has the disorder. This will need further testing with diagnostic tests for confirmation of the disorder.
- A negative screen test result means that the fetus is at lower risk of having the disorder as compared with the general population but it does not rule out the possibility that your fetus might have the disorder.