Every individual has 23 pairs of chromosomes (46 in total)–half from the mother and half from the father, 23 from each. Aneuploidy is a genetic (hereditary) disorder in which the chromosomes present are more or less than 46 in number. Genetic disorders can result from the presence of an extra chromosome copy (trisomy 47) or the lack of one chromosomal copy (monosomy 45) in the foetus. Variations in the chromosome number can have an impact on pregnancy, frequently resulting in miscarriages. 

For the past two decades, the need for prenatal aneuploidy screening has increased. Regardless of maternal age, aneuploidy screening is routinely recommended for all pregnant women to identify the risk of having chromosomal abnormalities such as trisomy 21 (Downs syndrome), trisomy 18 (Edward syndrome), and open neural tube defects (ONTDs). Several biochemical and ultrasonography screening tests are available in both the first (conception to 12 weeks) and second (12 to 24 weeks) trimesters of pregnancy for the early detection of chromosomal abnormalities.

Aneuploidy Screening

Aneuploidy screening is aimed to prevent the risk of a baby being born with genetic disorders primarily with open neural tube defects (ONTDs) and chromosomal abnormalities.

First-trimester prenatal screening: These screening tests in the first trimester of pregnancy measure the levels of β-hCG and pregnancy-associated plasma protein-A (PAPP-A) and an ultrasonography is performed to measure the nuchal translucency (NT).

Second-trimester prenatal screening: The maternal serum marker test is also known to be a “triple marker test” where the levels of alpha-fetoprotein (AFP), human chronic gonadotropin hormone (hCG), and estriol hormone (uE3) are measured, and in a “quadruple screen”, one more substance called inhibin-A is also tested. This screening test is prescribed to all pregnant women during 16-22 weeks, regardless of age, to identify a certain chromosomal disorder, such as Down syndrome and Edward syndrome. Additionally, neural tube defects, which are significant abnormalities of the brain or spinal cord, can be detected using this test.

AFP (Alpha-fetoprotein) Screening: 

This test measures the amount of AFP present in the mother's blood during pregnancy. The foetal liver produces AFP, a protein which is found in the amniotic fluid that surrounds the foetus. It then passes through the placenta and into the mother's bloodstream. Abnormal levels of this protein may indicate Down syndrome, spina bifida or other chromosomal disorders.

Human chorionic gonadotropin: During the early stages of pregnancy, the placenta produces the human chorionic gonadotropin hormone. Abnormally high levels of hCG indicate an increased risk of some chromosomal abnormality in the foetus.

Estriol: It is a hormone produced by the placenta and its levels can be measured by taking a blood or urine sample. It helps in determining the genetic health of the foetus.

Inhibin-A: This hormone is also produced by the placenta and it regulates the implantation and differentiation of the embryo.

Aneuploidy biochemical screening

The prenatal aneuploidy biochemical screening test is done to detect neural tube defects and chromosomal disorders in the foetus and is combined with NIPT screening to get detailed results.

Non-invasive prenatal Screening and Interpretation

NIPT testing looks at the DNA strands that are floating around freely in a pregnant woman's blood. These DNA fragments, also known as cell-free DNA (cfDNA), can be examined to detect any abnormalities in the developing foetus. This test is performed during the 10th week of pregnancy. Turner syndrome, Down syndrome, Edward syndrome, and Patau syndrome can all be detected with NIPT.

• A "positive" test result suggests that the baby might have an increased risk of having chromosomal disorders. It may require extra testing to confirm the diagnosis.  

• A "negative" result suggests that the baby has a decreased risk of having chromosomal disorders.

Prenatal tests are vital as they provide valuable information about the health of your baby. Even though these prenatal screening tests are taken based on an individual’s preference, they are made available to all women to reduce the risk of miscarriage and genetic defects in the unborn child. One can seek the appropriate guidance from a genetic counsellor based on the results of these tests. Taking wise decisions, guided by your healthcare provider, will go a long way towards protecting you and your baby from any untoward pregnancy outcomes.