Prenatal care helps reduce risks in pregnancy and ensures a safe and healthy delivery. One of the alarming issues related to pregnancy is aneuploidy. Aneuploidy is a genetic disorder in which the total number of chromosomes is more or less than normal, i.e., a total of 46 chromosomes or 23 pairs. In order to detect fetal chromosomal abnormalities at an early stage of pregnancy, it is important that the woman undergo prenatal aneuploidy screening. This test helps not just in the detection of any chromosomal aberrations but also in helping the individual decide whether or not to continue with the pregnancy.
What is fetal chromosomal aneuploidy?
Basically, aneuploidy is of two types: there can be an additional or extra copy of a chromosome (trisomy) or a deleted or missing copy of a chromosome (monosomy). When a fetus has either one of these conditions, it results in certain medical conditions. Trisomy can lead to Down syndrome, Edwards syndrome (Trisomy 18), or Patau syndrome (Trisomy 13). Monosomy can result in a condition called Turner syndrome.
These changes in chromosome number can significantly impact the outcome of pregnancy. In most cases, it often results in miscarriage during the first trimester of pregnancy itself. The best way to safeguard both the mother and fetus is to undergo an aneuploidy screening as early as possible.
Types of prenatal testing
Prenatal testing can be divided into two categories:
• Screening tests
• Diagnostic tests
Screening tests: These tests are done to find out if the baby has some birth defects, which are mostly genetic in nature. Screening tests include an ultrasound, blood tests, and prenatal cell-free DNA screening. These tests should be performed as early as the first or second trimester of pregnancy. In the case of any abnormality, further evaluation may be needed to confirm the diagnosis, which can help the mother decide whether to continue or terminate the pregnancy.
Diagnostic tests: These tests are done to confirm if the baby has an abnormality. While opting for an invasive prenatal diagnostic test, 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.
First trimester aneuploidy screening
Screening for chromosomal anomalies during the first trimester can help in the faster diagnosis of any genetic defects in the fetus. The maternal serum screening test is a blood test that can be performed at around 10 weeks (combined first trimester screening test) in a pregnant woman. The combined first trimester screening test can identify increased chances of the fetus suffering from any aneuploidies. This is combined with ultrasound around 11 and 13 weeks of gestation.
The maternal serum screening blood test measures the amount of two different proteins that occur naturally in a pregnant woman’s blood. This blood test is of two different types:
• Beta-human chorionic gonadotropin (ꞵ-hCG): A pregnancy-related hormone produced by the placenta during the early stages of pregnancy, human chorionic gonadotropin helps in maintaining pregnancy. When its levels are abnormally high, it may indicate an increased risk of some chromosomal abnormality in the fetus.
• Pregnancy-associated plasma protein A (PAPP-A): This protein is found in the mother’s blood during early pregnancy, and at high levels, it may indicate some form of chromosomal abnormality in the fetus.
The ꞵ-hCG hormone and PAPP-A protein, along with an ultrasound examination, are called dual markers or aneuploidy markers in the first trimester of pregnancy. The ultrasound test is known as nuchal translucency (NT), which is done to measure the thickening at the back of the fetal neck. An increased thickening may indicate the chances of the fetus suffering from Down syndrome, as nuchal translucency measurement is greater than usual in this syndrome. In addition to these tests, it is vital that a pregnant woman also undergo a NIPT test.
Aneuploidy markers normal range
While the normal range of ꞵ-hCG is 25,700–288,000 mIU/ml and that of PAPP-A is 1 MoM for all age groups. The test results can indicate whether the fetus has a low, moderate, or high risk of developing a chromosomal abnormality. A low-risk result means the absence of chromosomal abnormalities, however, in some cases, it does not necessarily rule out that the fetus may have an abnormality. In cases where the results are moderate to high-risk, it may require further testing, such as NIPT, to confirm the condition.
NIPT test in pregnancy
NIPT stands for non-invasive prenatal testing (also called non-invasive prenatal screening or NIPS), which is an advanced non-invasive blood test that helps identify and examine small fragments of free-floating DNA in a pregnant woman’s blood. These fragments are called cell-free DNA (cfDNA), and they are used to detect fetal abnormalities. This test can be done as early as 10 weeks of pregnancy. NIPT is effective in ruling out whether the fetus suffers from Turner syndrome, Down syndrome, Edward syndrome, or Patau syndrome.
The NIPT test report can either be positive or negative.
• "Positive" test result: A positive result may suggest that the baby might have an increased risk of having chromosomal disorders. It may require extra testing to confirm the diagnosis.
• "Negative" test result: A negative result may suggest that the chances of the baby having any chromosomal disorders are less.
The above aneuploidy screening test procedures are recommended for all pregnant women, especially those at high-risk (above 35 years of age or having a family history of birth defects). To summarize briefly, the double marker test acts as an important tool in the early detection of chromosomal abnormalities during pregnancy. It should be made an indispensable part of maternal screening for the safety of the mother and child and not be missed.