Pregnancy is a joyous occasion in the life of a woman. However, it can lead to anxiety concerning the unborn baby’s health. Most women give birth to healthy babies but in some cases, there may be chances that the baby can inherit genetic or congenital abnormalities. In order to protect the unborn child from such birth defects, it is advisable for a pregnant woman to undergo certain prenatal screening tests.
Types of prenatal testing
Prenatal testing can be broadly divided into two types:
- Screening tests
- Diagnostic tests
Screening tests: These include preliminary tests to find out if the baby has some birth defects, which may mostly be genetic in nature. Screening tests include a specific type of ultrasound, blood tests, and prenatal cell-free DNA screening. These tests are usually performed during the first or second trimester of pregnancy for early detection. In case of any abnormality, further evaluation may be needed to confirm the diagnosis.
Diagnostic tests: These tests are done in case there is an indication of the baby having an abnormality. Factors such as age, family history of genetic defects, or medical history of the pregnant woman that could put the baby at risk are taken into consideration when opting for an invasive prenatal diagnostic test. However, these diagnostic tests such as amniocentesis and chorionic villus sampling can result in slight chances of having a miscarriage.
Importance of prenatal testing
Prenatal tests are usually done to find out if the unborn child has any congenital defects or genetic disorders.
- Birth defects: These are structural changes that are present at the time of birth, affecting almost any part of the body. These changes may affect the way the body looks or functions. Some common examples include cleft lip, congenital heart defects, clubfoot, spina bifida, etc.
- Genetic disorders: Changes in the gene or chromosome of the baby can lead to genetic disorders. A gene is the structural and functional unit of inheritance while a chromosome is a cellular component that contains genes. Examples of genetic disorders are thalassemia, sickle cell disease, cystic fibrosis (CF), etc. Some chromosomal abnormalities are Down syndrome (Trisomy 21), Patau syndrome (Trisomy 13), Edward syndrome (Trisomy 18), etc.
What are maternal markers?
During the early stages of pregnancy, all women are offered screening of several maternal markers that indicate whether the fetus runs a risk of having certain birth defects and genetic diseases. The mother’s blood is screened for maternal serum biochemical markers such as alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), and unconjugated estriol.
Prenatal screening tests
• First trimester prenatal screening: This includes a combination of maternal blood testing and fetal ultrasound. These tests provide information about the risk of the fetus developing certain birth defects. Some of the screening tests included in the first trimester of pregnancy are as follows:
1. Maternal blood tests- These are further classified into 2 types of tests:
A. Human chorionic gonadotropin: This hormone is produced by the placenta during the early stages of pregnancy. At abnormally high levels, it can indicate the increased risk of some chromosomal abnormality in the fetus.
B. Pregnancy-associated plasma protein A: This protein is present during early pregnancy and a high level indicates chromosomal anomaly.
2. Ultrasound for fetal nuchal translucency- This screening test measures the area at the back of the fetal neck for increased thickening. It may indicate Down syndrome as nuchal translucency measurement is greater than usual in this syndrome.
• Second trimester prenatal screening: It may include a series of blood tests called multiple markers Which provide information about the risk of your baby developing certain birth defects or genetic conditions. As part of the prenatal genetic testing during the second trimester, following tests are done:
1. AFP (Alpha-fetoprotein) Screening- This test measures the level of AFP in the mother’s blood during pregnancy. AFP is a protein that is produced by the fetal liver and is present in the amniotic fluid, surrounding the fetus. From there, it crosses the placenta and gets into the maternal bloodstream. Abnormal levels of this protein may indicate:
A. Down syndrome or other chromosomal disorders
B. Defects in the abdominal wall of the unborn baby
C. Spina bifida and other similar neural tube defects
2. Estriol- This hormone is produced by the placenta and its levels can be measured by taking a blood or urine sample. It helps in determining the fetal health.
• Prenatal cell-free DNA screening: It is a type of blood test that examines traces of fetal DNA in the mother’s blood and helps in screening for specific chromosomal abnormalities such as Down syndrome.
Non-invasive prenatal screening and assessment
There are a few non-invasive prenatal screening tests that provide information about the risk of the fetus inheriting any genetic condition. Non-Invasive Prenatal Testing (NIPT) is an advanced non-invasive blood test that examines small fragments of free-floating DNA in a pregnant woman’s blood. These DNA fragments are called cell-free DNA (cfDNA) and they can be analyzed to detect fetal abnormalities. This test can be done as early as 10 weeks of pregnancy. NIPT is effective for the screening of Turner syndrome, Down syndrome, Edward syndrome, and Patau syndrome.
Pros and Cons of prenatal screening tests
Prenatal screening tests are beneficial to women over the age of 35 years as the chances of giving birth to babies with genetic abnormalities such as Down syndrome increases in older women. It prepares a family to decide whether to terminate the pregnancy or continue with it in case the fetus has some birth or genetic defect. If the family decides to have the baby, it helps them to prepare emotionally and mentally to provide the child with necessary care and attention, once the baby is born.
On the other hand, these prenatal screening tests are not confirmatory. They need to be further evaluated with diagnostic tests such as chorionic villus sampling and amniocentesis. The sensitivity of a prenatal screening test depends on the type of test a woman has undergone. In some cases, it will be only 80-95% accurate as there may be chances of getting a false positive result.
Finally, it may be said that while these prenatal screening tests are taken voluntarily, it should be offered to all women to limit the chances of birth and genetic defects in the unborn child. Based on the results of these tests, one can consult a genetic counselor for proper advice.
