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Genetic Testing During Pregnancy
Genetic testing during pregnancy usually involves analyzing blood or a tissue sample for studying missing or defective genes in the fetus.
Genetic testing also known as DNA – based tests is a new revolution in itself as it directly involves studies related to the DNA molecule using sophisticated techniques for identifying genetic disorders.
What are the diseases diagnosed by prenatal genetic testing
Prenatal genetic testing is aimed at diagnosing specific genetic disorders in the fetus. The diseases that are diagnosed by prenatal genetic testing include the following :
- Chromosomal aberrations
- Down syndrome
- Monogenetic diseases
- Polygenetic / multifactorial diseases
- Spina bifida
- Cleft palate
- Tay Sachs disease
- Sickle cell anemia
- Cystic fibrosis
- Muscular dystrophy
- Fragile X syndrome
What are the different tests for genetic testing
There are a variety of tests available for genetic testing. These tests are safe and do not cause harm to the baby. The tests are divided into 2 categories :
Screening tests basically assess the risk factor of having a genetic disability or abnormality. It needs to be clearly understood that, these tests will only estimate the “risk” of having a genetic disease and in no way confirm that the baby will be born with a particular disorder. The results of these tests will be produced as “high risk” and “low risk”.
It needs to be specifically mentioned here that, women who are classified as “high risk” may deliver a healthy and normal baby; and on the other hand those who are labeled as “low risk” may give birth to a baby with genetic defects. Therefore it is necessary that a confirmatory test should be carried out once the screening tests are done. The various screening tests include the following:
- Nuchal Translucency - This test is used for assessing the risk of baby who can take birth with a chromosomal abnormality. It is mainly done for examining Down syndrome, which is also known as trisomy 21. The test is generally done in the 11th week of pregnancy, but must be performed before the beginning of the 13th week. Such tests give more accurate results when performed in the 11th week + 3 days. For this test, an ultrasound is done for measuring the fluid filled sac that is present at the back of the baby’s neck. Babies with Down syndrome have an extra chromosome. Such children generally have intellectual disability, physical problems and certain specific facial characteristics.
Nuchal translucency test is often combined with a blood test for measuring the levels of 2 proteins in the women’s blood. This will give more accurate information about the chromosomal abnormality.
- AFP blood test – The AFP blood test is done for assessing the risk of baby who can take birth with a neural tube defect such as spina bifida. This test is done in between 16 – 18 weeks of pregnancy. Blood test done after the 18th week will not give any accurate information. Ultrasound in conjugation with AFP blood test will provide more reliable information.
- Triple test – This test is done for assessing the risk of baby be born with Down syndrome. This blood test measures the 2 maternal protein along with the human chorionic gonadotrophin (HCG) and unconjugated oestriol. These 2 hormone levels are combined with woman’s age and the risk calculated for having Down syndrome. This test is generally done in between 14 to 21 weeks. The test results are produced as “low” or “screen negative”, which means that the baby either has low or no risk of being born with Down syndrome. Tests results showing “high” or “screen positive”, mean that the chances of the baby being born with Down syndrome are higher.
Diagnostic tests make use of a sample from the placenta or the amniotic fluid and the cells are kept for culture for several days. Cell culture is a must in this case, as this procedure allows the baby’s genes and chromosomes to be mapped. The several diagnostics tests include:
- Chorionic Villus Sampling (CVS) - This is an invasive procedure as it requires the sample of placenta for studying genetic problems in the fetus. This test is generally done between the 10th and 12th week of pregnancy. CVS only provides information about genetic defects and not neural tube defects. It is therefore recommended that a blood test be done in the 16th – 18th week of pregnancy for diagnosing neural tube defects. There are 2 ways of performing CVS. One way involves insertion of cathether through the vagina to reach the woman’s cervix for obtaining the tissue. The second method involves inserting a needle through the abdomen to reach the woman’s uterus for obtaining the sample. The results are obtained within 10 – 14 days. In certain cases, the test gets prohibited due to presence of active vaginal infection.
- Amniocentesis – This test is used for diagnosing chromosomal abnormalities as well as neural tube defects. For this test, amniotic fluid is drawn from the woman’s abdomen for examination. The amniotic fluid contains the cells shed by the fetus, which is used for studying the genetic information of the baby. Amniocentesis is generally carried out between the 15th – 20th weeks of pregnancy and is recommended for women who are above 35 years of age. This procedure has some risk of miscarriage associated with it.
- Cordocentesis – This method involves direct analysis of the fetal blood for studying genetic defects like Down syndrome and blood disorders like haemophilia. Cordocentesis. It is done by insertion of a thin needle in the abdomen of the woman to reach the uterus and finally the umbilical cord for drawing fetal blood. The doctor constantly keeps monitoring the entire procedure through ultrasound images and the fetal heart beat is recorded simultaneously. The entire procedure takes about an hour and it may even cause discomfort to the woman. Once the tests are done women are advised to take complete rest for the next 24 hours to avoid complications such as vaginal bleeding and leaking of fluid from the amniotic sac.
Genetic testing during the second trimester of pregnancy
Genetic testing during the second trimester involves several blood tests known as multiple markers to assess the risk of having baby with genetic defect. The tests are usually done in the 15th – 20th week of pregnancy, wherein the maternal blood is tested for levels of proteins and hormones in her blood. It needs to be understood that these are screening tests and not diagnostic tests and are not 100% accurate. These tests include:
1. AFP screening – This test is done for measuring the levels of alpha - fetoproteins in the maternal blood. AFP is produced by the fetal liver and is present in the amniotic fluid, which crosses the placenta and reaches the mother’s blood. Abnormal levels of this protein may be the warning signs of the following condition :
- Down syndrome
- Neural tube defects like spina bifida
- Chromosomal abnormalities
- Abnormalities in the abdominal wall of the fetus
- Multiple fetuses
- Wrongly calculated due date, as the level of protein will be different for each trimester of pregnancy
2. hCG – This is a hormone produced by the placenta.
3. Estriol - This is a hormone produced by the placenta.
4. Inhibin - This is a hormone produced by the placenta.
Accuracy of genetic testing during pregnancy
Accuracy of the test results of genetic testing is a common question. Many couples want to know whether they should rely on the test results or should they go for some further confirmatory tests. Genetic testing generally has very good accuracy rate; however there are certain cases where contamination of the sample may lead to misdiagnosis.
Therefore it is always recommended that the samples should always pass the contamination check. Genetic testing for chromosome syndrome and single – genes disorders have an excellent accuracy rate.
Pros and cons of genetic testing during pregnancy
As the age of mother increases, her risk of having babies with Down syndrome and other chromosomal disorders also increases. All pregnant ladies over 35 years of age are advised to get genetic testing done during pregnancy. They are also offered genetic counseling, wherein a trained staff gives them all appropriate information regarding genetic diseases and help them in interpretation of the test results.
According to the American Congress of Obstetricians and Gynaecologists, all pregnant women should be offered genetic testing regardless of their age. There are several pros and cons that follow genetic testing.
Pros include – first hand information of the risk that the fetus may be carrying like Down syndrome or any other genetic disorder, and then taking a decision on whether to abort or go ahead with the pregnancy.
On the other side - several genetic testing procedures may also prove to be dangerous for some pregnant mothers. Tests like chorionic vilus sampling may put the woman at risk of developing infection, bleeding, premature rupture of membranes and subsequently loss of pregnancy.
Genetic counseling is a process by which the mother and her relatives are given all the necessary information about genetic diseases.
Genetic counseling should be given to each woman who opts for genetic testing of the fetus. Genetic counseling should be given both before and after the test for careful interpretation of the results. The German Testing Act has formulated the following points that must be covered during each genetic counseling session.
- Background information on several risks associated with congenital disease
- Individual risk associated with development of congenital disease
- Various options and limitations of genetic testing
- The various diseases that can be detected with genetic testing
- Risks and complications associated with different tests
- Conflictual areas with regards to prenatal diagnosis
- Several alternatives