Prenatal Diagnosis- An Overview

Prenatal diagnosis


A genetic disorder can be defined as the abnormalities in genome, whether it is by the process of alteration in the gene or a set of genes creating the genetic disorder. It is usually a congenital disorder (present from birth). Most genetic disorders are usually very rare and affect one person in every several thousands or millions of people.

The 4 comprehensive groups of genetic disorders are:

  • Single gene disorders (Sickle cell Anemia).
  • Chromosome abnormalities (Down Syndrome).
  • Multifactorial disorders (Diabetes, heart diseases).
  • Mitochondrial disorders (Leigh syndrome).

Among them, single gene disorders (a single gene causing a disease) and chromosome disorders (imbalance in the set of chromosomes) are predominant in every aspect. The 4 foremost ways of inheriting an altered gene are:

  • Autosomal dominant (50% risk) (Huntington disease).
  • Autosomal recessive (25% risk) (Sickle cell anemia).
  • X-linked dominant (50% risk) (Incontinentia pigmenti).
  • X-linked recessive (50% risk) (Duchenne Muscular dystrophy).

Prenatal diagnosis – An introduction

Parental diagnosis is the process of ruling in or out anomalies or genetic disorders in the foetus. It aims to provide genetic information about the foetus to the expecting parents and also the opportunity to modify pregnancy management and/or postnatal care. Some screening detects the diseases that affect the health of a mother while, some diseases detect the overall health of the zygote, embryo, or foetus, either before the gestation even starts.

The testing and screening depend on which trimester the baby is in. The prime purpose of prenatal diagnosis is – To provide timely aide for the mother or the baby, to give parents some time to abort the foetus if the condition is too severe or, to give parents socially, psychologically and emotionally to adjust to the situation accordingly.

Under what circumstances is the prenatal diagnosis required?

  • Mothers over the age of 30 (High chance of Down’s syndrome).
  • Pedigree analysis with long family history of genetic disorders. (Example-Family history of cleft lip or palate, spina bifida, congenital heart defects).
  • Multiple still births, pregnancy loses.
  • Marriage between two related individuals (cousins).
  • Increased intake of Alcohol, Nicotine, Cocaine (disrupts brain development and nerve tissue).

We will now ponder over few techniques to diagnose genetic defects in fetuses:

Chorionic Villus Sampling (CVS)

It is a prenatal diagnostic procedure that uses fetal chromosomes to yield and find single gene disorder and lay out the assay for the biochemical disease. It provides an overview about the embryonic karyotype. It is usually performed between the 10-12 weeks of pregnancy. As both placenta and embryo arise from the same cluster of totipotent cells, therefore, depending upon the position of placenta at the particular time, samples of trophoblast and chorionic villi are taken trans abdominally or Trans cervically.


It is a technique for the diagnosing of any fetal abnormality using amniotic fluid as a sample. Amniotic fluid has exfoliated fetal cells that are cultured to reveal fetal karyotype, and/or are used to perform biochemical testing and molecular analysis. It is usually carried out at 16-18 weeks of pregnancy.


If suspended, fetal cells in amniotic fluid contains high levels of alpha-fetoprotein (AFP) used to screen for neural tube and abdominal wall defects (gastroschisis). However, if there is a presence of (AChE) acetylcholinesterase in amniotic fluid, then it may indicate an open neural tube or ventral wall defect is present.

Percutaneous umbilical blood sampling (PUBS) also known as (Cordocentesis)

It is a method of obtaining fetal blood from the umbilical vein under ultrasound Guidance. During this procedure a needle is inserted through the mother’s abdomen to obtain a fetal blood sample (lymphocytes). It is used to diagnose fetal toxoplasmosis and haematological ailments. This is done by measuring increases or decreases of particular blood factors in foetus. It also used to diagnose chromosome instability such as Fanconi anaemia, prenatal infections and aneuploidy. It is also used for in utero blood transfusion It is usually performed when foetus is 18-20 weeks of pregnancy.

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