Healthy Pregnancy: Prenatal Screening For Chromosomal Anomalies In Expecting Mothers

Healthy Pregnancy: Prenatal Screening For Chromosomal Anomalies In Expecting Mothers
The aim of prenatal care is to predict potential complications early in pregnancy so that the obstetrician and the parents are well prepared.

Prenatal screening can predict potential complications early in pregnancy so that you and your doctor man immediately take corrective measures.

Written by Editorial Team |Published : July 31, 2021 5:44 PM IST

The moment a woman sees a "positive result" in the urine pregnancy test, her dream journey of being a fulfilling mother starts. Such are the strong emotions of being pregnant and having a successful journey. Today, we are living in an era of "small families" and everything going "perfect", and pregnancy indeed tops the list for "perfect endings". Despite the advances in medical science to make pregnancies safer than ever before, some cases can still run into trouble, either with the mother or the baby and sometimes, both.


The aim of prenatal care is to predict potential complications early in pregnancy so that the obstetrician and the parents are well prepared and, in some instances, can take action to prevent or reduce the severity of the complications. The most common concerns during pregnancy affecting the mother are development of high blood pressure, called preeclampsia (PE) or pregnancy related hypertension (PIH) and gestational diabetes. With respect to fetal development, the common concerns are genetic or structural anomalies, early or preterm delivery, and fetal growth restriction leading to short-term and long-term problems for the child. When screened during early pregnancy, these complications can be managed better. Hence, a pregnant woman must be aware of screening tests, their accuracy, and the available options if found to be at an increased risk.


A screening test will tell us the possibility of "being affected". Screening tests must be offered universally, i.e., to all relevant individuals to get the best possible results for a population. A good screening test has a high detection rate/ accuracy and a low "false positive rate". The prenatal screening tests involve a combination of blood test and fetal scan for best prediction. A "screening test" will report as "increased/ high risk" which means this mother needs to be monitored more closely or offered further confirmatory tests or a "low risk", which is reassuring. A third category of "intermediate risk" prompts further tests to improve the detection rate.

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Every pregnant mother is "at risk" of having a baby with a chromosomal anomaly. The most common non-inherited cause for congenital mental challenge is Down Syndrome (DS) which is caused due to the presence of an extra chromosome. Instead of 23 pairs, i.e., 46 chromosomes, the individual has an extra or partial copy of chromosome 21. Apart from its distinctive facial appearance, DS can result in many physical and mental developmental disorders.

Screening tests are designed to detect such chromosomal anomalies. The accuracy of each test is variable and depends on the standards maintained by the laboratories and scan operators.


Below are some of the tests that all pregnant women must take to ensure a healthy pregnancy.

Cell free DNA testing for Trisomy 21 (DS) (cfDNA test)

This test has a 99 per cent accuracy in predicting the possibility of DS and a 95-98 per cent accuracy for other chromosomal anomalies. This is a blood test performed on the mother. It is combined with a fetal scan for accuracy. The accuracy of the test depends upon the amount of "fetal and placental" cells secreted in the mother's blood and hence can never be 100 per cent accurate. A high-risk result is confirmed with an amniocentesis. This test has a low false-positive rate of 1 per cent. Therefore, confirmatory testing after a "high risk" or "no result" is mandatory, especially, if the fetal scan is normal.

The first trimester Combined test(1T-CT)

It includes a fetal scan at 11+0 13+6 weeks (popularly known as Nuchal Translucency Scan, or NT scan) and maternal blood test which includes 2 or 3 hormones (beta hCG, PAPP-A and/ or PlGF), which are pregnancy specific. When performed at the right time, this has an accuracy of around 90-92%. A low-risk result is reassuring, but not confirmatory, i.e., it is 'low' risk but not 'no risk'. In case of high-risk result, further diagnostic test with an invasive procedure or screening with cfDNA testing is offered. More recently, a concept of "intermediate risk" has been introduced, which is between 1:251 and 1:1000, where further testing is offered to women who want a better detection rate or a diagnostic test.

Sequential test (1T CT + 2T Quad marker + Genetic sonogram)

This test combines the 1T combined adjusted risk with the 2nd trimester quadruple marker test and genetic sonogram, the latter being a part of the anomaly scan. The final adjusted risk depends upon the hormone profile of the 2T Quad test, presence, or absence of "soft markers", and the fetal defects in the anomaly scan. Based on the final adjusted risk, parents may be counselled to choose between invasive tests (amniocentesis after 16 weeks) or carry on without any further testing. The detection rate of sequential testing is up to 95 per cent. However, the accuracy depends on the standard of the 2nd trimester fetal anomaly scan.


The standards maintained by a screening lab are critical when it comes to accuracy of results. It must confer to international standards set by professional bodies like, National Accreditation Board for Testing and Calibration Laboratories (NABL) and UK National External Quality Assessment Service (UKNEQAS). They must be performing these tests regularly on standardized analyzer platforms under regular audits. Most importantly, the staff must be trained on the prenatal screening techniques and the importance of taking a good, detailed pregnancy history as factors like maternal age, weight, BMI, type of conception, maternal diabetes, twins, fetal gestational age etc. affect the hormone profile.

As a pregnant mother, it is vital to understand the advantages and limitations of each test when undergoing prenatal screening. It is equally important to choose the right laboratory and trained doctors to perform the prenatal scan. At the same time, one must keep in mind that despite a doctor's sincere and well-meaning efforts, some pregnancies can take an unexpected turn. But, staying aware and educated can help one make informed decisions in consultation with the obstetrician, thereby improving the chances of a healthy pregnancy.

(This article is authored by Dr Prathima Radhakrishnan FRCOG, Scientific Advisor, Lilac Insights Pvt Ltd, Director & Consultant in Fetal Medicine, Bangalore Fetal Medicine Centre)

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