Diagnostic tests like Amniocentesis and Chorionic Villus Sampling are done to confirm foetal abnormalities, after it has been found through screening tests that you are at a risk of producing an abnormal baby. Even a direct diagnostic test may be offered, instead of screening tests if you have a greater risk of producing a baby with birth defect.Your risk of having a defective baby is high, if you have a personal or family history of birth defects, if your previous child had a birth defect, you were diabetic before getting pregnant or used certain medicines around the time of conception.
These tests confirm whether the foetus shows any abnormality or not. Tests commonly offered to diagnose abnormalities in the foetus - Amniocentesis and Chorionic Villus sampling.
Is usually performed in the second trimester between 16 to 18 weeks or anywhere between 11 to 14 weeks of pregnancy during the first trimester under ultrasound guidance. Nearly 99 percent of all chromosomal abnormalities can be detected with an amniocentesis test. During this test, a small amount of amniotic fluid present around the foetus is withdrawn, the cells from the fluid are separated and cultured. Then chromosomes or genes are extracted from the cultured cells and analysed for possible defects. Most chromosomal and genetic disorders such as Down syndrome, cystic fibrosis and haemophilia, and neuraltube defects can be detected with the help of amniocentesis test.
However, the test carries a small risk of causing a miscarriage in early pregnancy, and the risk of causing uterine infection in the days following the procedure, and therefore the test is administered as an option to women, in whom risk for birth defects is high. Women of age 35 and above, those with a family history of birth defects and who have had a previous child with some defect are at a greater risk, and such category of women can take this test. The second trimester amniocentesis has a lesser risk than the one done during the first trimester because there is more amniotic fluid around the baby, in the second trimester, and the procedure of withdrawing a little of the fluid will not harm the baby.
Chorionic Villi Sampling
This test can be done in early pregnancy, at about 10 weeks. It can detect chromosomal and genetic disorders similar to amniocentesis test. Chorionic villi are fingerlike outgrowths on the placenta, and these are genetically alike with the foetus, so analyzing the villi will reveal the abnormalities present in the baby. A sample of choriionic villi is collected, cultured and studied under a microscope to check for chromosomal defects. Inborn errors in enzyme function can also be diagnosed with this test. Muscular dystrophy, a condition mostly related to a male baby can also be detected. Since the test can be done in early pregnancy, you get to know about any abnormality present in your baby early.
Knowing the presence of abnormality early will help you to decide even at an early stage whether to continue with the pregnancy or terminate it, which will be much safer than deciding on termination at a later stage. This test has a slightly higher risk than amniocentesis, of miscarriage, as it is done as early as 10 weeks. It is not advisable to undergo this test before 10 weeks, because of the potential risk it carries.