This is a diagnostic test that is usually done if the maternal serum screening test (or the triple marker or AFP) done earlier, shows abnormal results. It is primarily used to detect Down's syndrome or other chromosome (genetic) abnormalities and neural tube defects such as spina bifida.
This test is very accurate (close to 100%) but only certain defects can be detected. It is possible, even after a negative test that the baby has birth defects and that even after a positive test, the baby does not have a birth defect when born---but the likelihood of these scenarios is very low.
Using the ultrasound as a guide, the doctor inserts a thin needle through the mother’s abdomen and uterus. A very small amount of amniotic fluid (the fluid that surrounds the baby in the uterus) is removed and tested. The doctor may check the baby’s heart beat to make sure its normal and that the baby is not in any distress because of the needle.
The test can be done in a hospital or in the doctor’s office. No overnight stay is required.
Your doctor will advise this test based on the following signals:
- If you have had an abnormal maternal serum screening test (AFP or triple marker)
- Are older than 35
- Have a family history of genetic disorders (or a partner who does)
- Have previous child with Down's syndrome or a neural tube defect
Between 15-20 weeks
Many times women are advised amniocentesis only if the AFP is abnormal and have to wait until after the results of AFP come. The test also needs to be done before 20 weeks (which is the latest a woman can go in for an abortion in India), so if the result points at birth defects then you can take a careful decision on whether to terminate the pregnancy in a timely manner or prepare for a special baby.
This procedure has a very small risk of miscarriage. Some women have cramping, spotting or leaking of amniotic fluid after the procedure. If these happen, you should call your doctor immediately. Usually, as a precaution, women are advised rest for a day or two to prevent miscarriage.
The person who conducts this test is usually a specialist doctor and it is important that this person be very skilled and reputable.
How much time does it take for the results?
Within 1 to 2 weeks
Pros and cons of Amniocentesis over CVS
Risk of miscarriage with amniocentesis is slightly less as compared to CVS especially since CVS is much earlier in the pregnancy. Also CVS, cannot detect neural tube defects.
CVS can be done earlier (10-12 weeks) than amniocentesis. So if there is a birth defect, it gives the parents time to make a decision about whether to terminate the pregnancy early on in the pregnancy.
Because of these reasons Amnio is more popular than CVS . For most cases, these days, usually a combination of maternal serum screen (triple screen or AFP) along with Amniocentesis is used for checking birth defects. If the likelihood of genetic defects (like Down's syndrome) is very high, only then doctors might advise CVS