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Prenatal genetic testing is done during pregnancy to diagnose any complications with the mother or developmental issues with the fetus. It can be controversial, but also beneficial. In past years, prenatal screening included amniocentesis and other invasive procedures. These could harm the fetus, and mothers were told that this could cause miscarriages.
But in recent years, new breakthroughs have improved the safety and reduced the invasiveness of prenatal screening. Fewer women need to undergo amniocentesis or chorionic villus sampling (CVS). The newest type of test measures fetal DNA fragments found in the mother's blood, and as a result, the test calls for a simple blood sample from the mother. However, these tests are recommended only for women with high-risk pregnancies under a doctor's order.
What happens in prenatal genetic screening?
Amniocentesis is one of the most common invasive prenatal tests. In this procedure, a sample of amniotic fluid, the fluid that surrounds the fetus, is drawn and analyzed. It is invasive and carries a small risk of miscarriage, so amniocentesis should only be done to diagnose intrauterine infections, to determine if an early delivery is advisable for medical reasons, or for women who have an increased risk of genetic and chromosomal problems.
Amniocentesis is used to detect nearly all chromosomal disorders and several hundred genetic disorders. It is done between weeks 16 and 20 of the pregnancy.
CVS is a similar test, except it can be done between 10-12 weeks of pregnancy. This test takes cells from the placenta and analyzes them for genetic abnormalities. The main difference is that CVS cannot detect any neural tube defects, while amniocentesis can.
These tests are usually done in conjunction with a genetic counselor, who can help you through the process of making the right choices and understanding what’s best for you and the baby.
Who needs genetic testing?
There are a number of risk factors that require screening. You may want to consider genetic testing if:
- You are 35 or older on the due date.
- You’ve had three or more miscarriages, or had a stillbirth in the past.
- You have an ethnic background that carries a higher risk for conditions.
- You’ve been exposed to substances or treatments, like chemicals or drugs, that could affect your baby’s well being.
- You take medications for certain conditions, like epilepsy or diabetes, and will continue to take the medication during pregnancy.
- You’ve had a positive result on the first trimester combined screening, which uses ultrasound and blood tests to screen for abnormalities.
- Genetic disorders run in the family for either you or your partner.
Invasive vs. non-invasive genetic testing
Genetic testing has come a long way in the past 20 years. First trimester screening combines a maternal blood test and ultrasound to look for chromosomal abnormalities. Fetal DNA can be isolated in maternal blood, so screening tests to identify major trisomies (Down syndrome, Edwards syndrome, Patau syndrome) are now available to high risk women without the risks of amniocentesis or CVS.
The pros of genetic testing
- Noninvasive screening tests can be offered as early as 10 weeks and let parents know if their child is at risk for the three trisomies above. Depending on the severity of the abnormality, the parents are able to make the right decision for themselves and decide whether or not to terminate the pregnancy.
- Invasive screening tests can be done as early as 10 weeks and can detect nearly all chromosomal disorders, as well as hundreds of chromosomal disorders at a very high accuracy rate.
- Knowing whether or not a baby will have special needs will help the parents plan accordingly. The house may need to be altered, or family members prepared for the birth. Knowing ahead of time will also help the mother plan the delivery more accordingly.
- Problems with the fetus may be able to be rectified before the baby is born, or at least prepare for a surgery as soon as the baby is born, if needed.
The cons of genetic testing
- The costs can be prohibitive. Noninvasive screens are not always covered by insurance and can cost anywhere from a few hundred to a few thousand dollars.
- Noninvasive screens are screens, not diagnostic tests. If the screens come back positive, a more invasive diagnostic test may need to be done. Noninvasive screens also only test for a few abnormalities, while invasive tests test for many more.
- With invasive tests, there is a small risk of injury to the baby or miscarriage.
- Genetic testing can cause a serious amount of anxiety, especially since screenings do not diagnose.