Prenatal diagnosis is the detection or exclusion of abnormality in the foetus prior to birth. In the process of having healthy progeny, prenatal diagnosis useful in detecting health of a baby to be born. It is useful to detect genetic disorders where the couple or populations are at high risk of inheriting genetic disorders.
If an abnormality is detected in the pregnancy, the parents can take decision as to whether to continue with the abnormal foetus or to go for abortion. Prenatal diagnosis enables:
- (1) Timely medical or surgical treatment of a condition before or after birth,
- (2) parents to abort a foetus with the diagnosed condition.
- (3) parents he to be prepared psychologically, socially, financially and medically for a baby with a health problem or disability or for likelihood of a still birth.
Prenatal diagnosis for genetic diseases is now commonly available for pregnancies at risk. Prior to prenatal diagnosis genetic counselling could only give likelihood of recurrence, based on Mendelian laws or empirical data. If an abnormality is detected in the pregnancy the medical people have to convey the information to parents and allow the parents to take a decision as to whether to continue with the abnormal foetus or to go for abortion. The couples at high risk of having a child with a genetic disorder has to choose between taking the risk or considering other reproductive options such as long term conception, sterilization and termination of pregnancies. Most parents would terminate an affected pregnancy but some choose to use the information to prepare for the birth of an affected child. When the foetus is found to be normal it allows the parents to continue the pregnancy with mental relief.
Many factors are to be considered before undertaking prenatal diagnosis, like risk of inheriting a genetic disorder, severity of genetic disease etc. Some genetic diseases lead to death in utero, or infancy or childhood. Some diseases are compatible with survival for many years with severe handicaps. Many genetic diseases cannot be cured even if the basis for the diseases is known. In some cases gene therapy may help. Currently prenatal diagnosis tests are available for more than 200 genetic disorders. Before going to a test one should be careful in finding the reliability of the test.
Indications for Prenatal Diagnosis
There are many indications for offering prenatal diagnosis. While undertaking prenatal diagnosis the risk of an abnormal foetus is at least as great as risk of the procedure itself. The following are some of the indications:
- Advanced maternal age: Women who are pregnant at 35 or more have higher risk of giving birth to a baby with the defect in chromosomes. Down syndrome (trisomy 21) is the common chromosomal defect. A child with Down syndrome can be born to parents of any age; however, the risk rises steeply once the mother’s age is over 35 years.
- Previous child with a chromosome abnormality: Parents who have had a child with trisomy 21 (Down syndrome) or any other trisomy are at increased risk of having a trisomy in subsequent pregnancies. Family history of a chromosome defect: A pregnant woman with a family history of chromosome defect are often advised for prenatal diagnosis. The most common abnormality is Down syndrome (Trisomy 21).
- Family history of a neural tube defect: If a first-degree relative of the foetus (parent or sib) has a neural tube defect, the risk to the foetus is in the 2 to 5 percent range. If more remote relatives are affected, the risk of the foetus is less but may still be above the population risk.
- Other indications for prenatal diagnosis include structural aberrations in a family or previous pregnancy, single gene genetic disorders, parental consanguinity, mental retardation etc.
Techniques of prenatal diagnosis: There are several techniques available for prenatal diagnosis. The following are some of the methods.
- Amniocentesis: In this technique amniotic fluid would be aspirated by inserting a needle through the abdomen into the mother’s amniotic cavity . This procedure is to be performed under ultrasound guidance. Amniocentesis is usually carried out between 14-16 weeks of gestation and 10-20ml of amniotic fluid is aspirated. The cells have to be cultured for an average of 8-14 days to obtain a foetal karyotype. The risk of the method appears very small; however there is a small risk of inducing miscarriage. With this method Karyotyping and some biochemical disorders can be detected.
- Chorionic villus sampling: This technique is carried out between 9 and 12 weeks of gestation and involves the removal of small amount of placenta. This technique is suitable for chromosome analysis following the determination of a high risk of chromosome abnormality. The advantage of this technique is early and definitive chromosomal analysis. This test bears a risk of pregnancy loss varying from 1 to 3 percent. In this technique karyotyping, DNA and biochemical disorders can be detected.
- Foetal blood sampling: This technique is also called cordocentesis. It involves the removal of small amount of foetal blood by inserting a small needle into the site where the cord joins the placenta. In this technique rapid karyotyping can be done.