What is Prenatal Diagnosis?

Prenatal diagnosis is a genetic diagnostic procedure important in detecting and preventing many Mendelian disorders. Significant advances since the mid-1980s have been the development of chorionic villus sampling procedures in the first trimester and the application of recombinant DNA techniques to develop.

Counselling to explain the consequences of prenatal diagnosis

I. Various prenatal procedures are available, generally being performed between 10 and 20 weeks’ gestation. Having prenatal tests and waiting for results is stressful for couples. Careful assessment of the attitudes of the patients is important to make whatever decision is taken, acceptable to the couple.

II. When a fetal abnormality is detected through prenatal diagnosis, decisions have to be taken whether to terminate the pregnancy or not. All couples whom elect for medical termination of pregnancy (MTP), following an abnormal test result need counselling and psychological support afterwards.

III. Or in case of no abnormality found, couple are needed to be reassured that there will be no difficulty after giving birth.

How prenatal diagnosis identifies risks?

Pregnancies at risk of fetal abnormality may be identified in various ways:

1. At Advanced maternal age, pregnancies may be at increased risk of developing Down syndrome or other chromosomal abnormality.

2. One of the parents with a familial chromosome translocation or congenital abnormalities has been identified by prenatal ultrasound scanning.

3. A high risk of a single gene disorder may have been identified through the birth of an affected relative.

4. Population screening programmes can identify the risk of couples from certain ethnic groups, whose pregnancies are at high risk of particular autosomal recessive disorders, such as the haemoglobinopathies or Tay–Sachs disease, before the birth of an affected child.

5. Screening for carriers of cystic fibrosis is also possible, but not generally undertaken on a population basis.

6. In many Mendelian disorders, particularly autosomal dominant disorders of late onset and X linked recessive disorders, family studies are needed to assess the risk to the pregnancy by prenatal diagnosis.

Methods of prenatal diagnosis

1. Maternal serum screening: Estimation of maternal serum α fetoprotein (AFP) concentration in the second trimester is valuable in screening for array of genetic abnormalities like: threatened abortion, neural tube defects, placental haemangioma, Turner’s syndrome etc.

2. Ultrasonography: Obstetric indications for ultrasonography are well established and include: confirmation of viable pregnancy, assessment of gestational age, localisation of the placenta, assessment of amniotic fluid volume and monitoring of fetal growth. It also structural abnormalities of the brain, various types of congenital heart disease, clefts of the lip and palate and microphthalmia.

3. Amniocentesis: Amniocentesis is a well-established and widely available method for prenatal diagnosis. It is usually performed at 15 to 16 weeks’ gestation but can be done a few weeks earlier in some cases. Amniocentesis indicates risk of Down syndrome or other chromosomal abnormalities and for estimating α fetoprotein concentration and acetylcholinesterase activity in amniotic fluid in pregnancies at increased risk of neural tube defects.

4. Chorionic villus sampling: Chorionic villus sampling is a technique in which fetally derived chorionic villus material is obtained trans-cervically with a flexible catheter between 10 and 12 weeks’ gestation or by trans-abdominal puncture and aspiration. The main indications for chorionic villus sampling include the diagnosis of chromosomal disorders from familial translocations and an increasing number of single gene disorders.

5. Fetal blood and tissue sampling: Fetal blood samples can be obtained directly from the umbilical cord under ultrasound guidance. Blood sampling enables rapid fetal karyotyping. Indications for fetal blood sampling to diagnose genetic disorders are decreasing with the increased application of DNA analysis performed on chorionic villus material.