What is non-invasive prenatal testing?
Non-invasive prenatal testing, also known as non-invasive prenatal diagnosis, is a screening method to detect certain genetic defects in the unborn baby. This is a sophisticated test which examines the genetic material of the mother (maternal DNA) to determine whether the growing baby is at risk of suffering from genetic diseases like Down syndrome (trisomy 21), Edwards syndrome (trisomy 18) or defects in the sex chromosomes (Turner syndrome).
This testing also helps to determine the blood group and sex of the baby. Non-invasive prenatal test is usually recommended in women with certain risk factors for carrying baby with genetic or other defects. The consulting doctor and genetic counselor helps the patient to explore the risks and benefits associated with prenatal testing and other decisions that may need to be taken.
Reasons for prenatal testing
Living with a genetic disease has a host of implications both for the child and the parents. Early diagnosis can allow the parents to make a decision about the viability of the pregnancy and prepare for the possibility of having a child with these genetic conditions. The non-invasive prenatal testing is more sensitive and specific in comparison to conventional first and second trimester screening tests. It is preferred over other invasive tests like chorionic villous sampling and amniocentesis in high risk mothers.
Amniocentesis is a procedure in which sample of amniotic fluid is collected from the amniotic sac protecting the developing baby in the uterus of the mother. Chorionic villous sampling is where a small amount of tissue from the placenta is collected for examination. Both of these procedures carry a small risk of miscarriage. Therefore noninvasive prenatal testing is recommended for pregnant women as there is no risk of miscarriage from carrying out this type of testing procedure.
Testing for high risk mothers
Some women are considered to be at a higher risk of carrying a baby with chromosomal defects. The common risk factors includes elderly mothers or a past history of having a baby with chromosomal abnormality as in Down syndrome, Edward syndrome or Turner syndrome. Sometimes the gynecologist may recommend non-invasive prenatal screening test if there is concerning results obtained from other screening tests like the quad test or first trimester screening test.
Testing is also advisable for mothers who are themselves carrier of some X linked recessive diseases namely Duchenne’s muscular dystrophy or hemophilia. These diseases typically affect male babies. The non-invasive prenatal test detects the baby’s sex earlier than an ultrasound examination. After knowing the sex of the baby the genetic counselor helps in managing the outcome of the pregnancy.
Testing should also be considered in mothers who have Rh negative blood group while the partners has Rh positive blood group. Knowing the blood group of the baby helps in management of the pregnancy. Babies with positive Rh group and a mother with negative Rh blood group may be problematic. There is a chance of antibody production by the maternal immune system which destroys the blood cells of the baby and leads to serious complications.
Before deciding on the test, the parents should consult with a genetic counselor and doctor about the outcome of the test. It is important for parents to understand the implications of the test results and how it may affect the pregnancy and their lives after the child is born. The test is performed around 10 weeks of pregnancy.
A blood sample from the mother is collected and sent for DNA analysis of both maternal and fetal genetic material. Usually the test results are available after 2 weeks of the test and sometimes sooner. After receiving the result, the genetic counselor and doctor interpret the results for the parents and discuss the implications, if any.