What genetic tests are done during pregnancy?
In the United States, almost 1 in 33 babies are born with birth defects each year which adds up to about 3% of all babies born in the United States. Some of these birth defects in babies are caused by genetic abnormalities. Genetic screening during pregnancy is thus important for families with the history of these birth defects or genetically inherited disorders.
Diagnosis of any genetic disorders that was inherited or has developed during pregnancy can be done through genetic tests during various stages of pregnancy. Earlier detection of genetic disorders such as chromosomal aberrations like trisomy 18, trisomy 21 (Down Syndrome), and birth defects like spina bifida can allow parents to be prepared for the medical costs and how to care for their child.
Types of genetic tests and genetic disorders during pregnancy
Genetic tests are divided into two categories, screening tests, and diagnostic tests. Genetic screening tests are blood tests done usually before pregnancy or in early pregnancy to determine the risk of your baby of inheriting a genetic condition. On the other hand, diagnostic tests are done during pregnancy that can identify and confirm any chromosomal anomalies in your baby.
Genetic screening tests are relatively safe for you and your baby as they only require a blood or saliva sample. Screening tests discussed today will include carrier screening, first-trimester screen, and multiple marker screening.
Carrier Screening Tests
Carrier screening tests are also done for single gene disorders such as cystic fibrosis, thalassemia, sickle cell anemia, and Tay – Sachs disease. This screening test can be used to check if both parents are carriers of a genetic mutation that may cause a possibility of their baby inheriting a genetic disorder.
Being carriers of the disease means that both parents are both heterozygous as they carry one dominant and one recessive allele of the gene. As a carrier, the recessive gene is not expressed therefore would not have the disease. This screening test is more commonly done as early as possible, either before pregnancy or early in the pregnancy.
The first-trimester screen involves a combination of an ultrasound of your baby’s nuchal fold (nuchal translucency) and a blood test. The blood test would include screening for genetic disorders such as Down Syndrome (trisomy 21), trisomy 18, and spina bifida (open neural tube defect) and other chromosomal anomalies.
Multiple marker screening (Triple or Quad screen)
Double marker screening or triple marker screening or quad marker screening test is done during the second trimester of pregnancy to assess the risk of the baby having genetic conditions like Down Syndrome, trisomy 18, and spina bifida. It is done by drawing blood from the mother and subsequently measured for the concentration of different components in the blood.
Measured components are alpha-fetoprotein (AFP), unconjugated estriol (uE3), human chorionic gonadotrophin (hCG), for a triple test. Inhibin A is included for a quad screen test. High levels of alpha-fetoprotein (AFP) could mean that you’re expecting more than one baby or it could be a sign of spina bifida (open neural tube).
However, low levels of alpha-fetoprotein (AFP) and unconjugated estriol, and high human chorionic gonadotrophin (hCG) and inhibin A could mean that your baby has the higher risk of Down Syndrome. As for other chromosomal aberrations such as trisomy 18 where your baby has an extra chromosome 18, low levels of AFP, estriol, and hCG could be associated with higher risk of them.
When a triple or quad screening is done following first – trimester screening tests it is known as integrated or sequential screening.
Genetic counseling may be suggested after any abnormal results arise from genetic screening to discuss any possible birth defects.
Unlike screening tests, genetic diagnostic tests will provide more concrete evidence of your baby having a genetic birth defect or not. These tests are invasive as a needle is inserted in the womb, thus have a small risk of miscarriage. Genetic diagnostic tests to be discussed will be chorionic villus sampling and amniocentesis. These are the only two tests that can diagnose genetic defects during pregnancy.
Chorionic Villus Sampling
Performed during the first trimester (10-12 weeks) of pregnancy, to determine the total chromosome number of the baby especially to check if your baby has the normal number of 46 chromosomes. This diagnostic test is advantageous because it is done earlier in the pregnancy to determine any chromosomal defects such as Down Syndrome (trisomy 21). The chorionic villi are finger-like projections in the placenta, a needle is inserted through the cervix or injected to get a sample of the cells. This sample can also test for multiple other genetic disorders like thalassemia, cystic fibrosis, and Tay – Sachs disease just to name a few.
This is a diagnostic test done during the second trimester (16-20 weeks) of pregnancy, where a sample of amniotic fluid is drawn from the womb. Amniocentesis can detect many genetic disorders that include trisomy 18, trisomy 13, Down Syndrome (trisomy 21), and other chromosomal abnormalities like Turner’s Syndrome and Klinefelter Syndrome. Other single gene diseases can also be detected through amniocenteses, such as sickle cell disease and cystic fibrosis. Neural tube defects such as spina bifida and anencephaly can also be detected.
Why are genetic tests important during pregnancy?
When there was any family history of genetic conditions or previous miscarriages you doctor may suggest having these genetic tests done. Diagnostic tests are also especially important if parents were tested positive carriers of a genetic condition.
Genetic screening for Down Syndrome is usually important for pregnant women aged 35 and older this is because the genetic risk of Down Syndrome increases with maternal age.
As with any form of medical tests, there will be contraindications and risks, always consult your doctor for advice before making any decisions to go ahead with it. One life lost is one too many especially when it comes to your own baby.