In amniocentesis, the doctor inserts a needle through the mother’s skin into the amniotic sac to collect about an ounce of amniotic fluid. The fluid contains genetic material from the baby that can be analyzed for signs of defects.

Most healthy women do not need all the tests. Ultrasound examinations during pregnancy are routine, although they are not always required and rarely influence treatment decisions. Amniocentesis and Chorionic Villus Sampling (CVS) are recommended only when a risk of genetic problems exists because of family history or something detected during an ultrasound. Amniocentesis and CVS carry a slight risk of harming the baby and mother, or ending the pregnancy in miscarriage, so those risks should be weighed carefully against the potential benefits of learning about the baby’s condition.

Amniocentesis, the removal and analysis of a small sample of foetal cells from the amniotic fluid, is widely available and involves a lower risk of miscarriage than chorionic villus sampling. However, amniocentesis cannot be done until the 14th to 18th week of pregnancy.

Chorionic villus sampling, conducted at 9 to 11 weeks of pregnancy, involves extracting a tiny amount of chorionic villi, tissue extensions that will eventually develop into a placenta. The villi can be obtained through the pregnant woman's abdomen or cervix. This type of sampling carries a 1-2% risk of miscarriage.

A third diagnostic method, percutaneous umbilical blood sampling or PUBS, is the most accurate method in detecting Down's syndrome and can be used to confirm the results of CVS or amniocentesis. However, PUBS cannot be performed until later in the pregnancy, during the 18th to 22nd weeks, and carries the greatest risk of miscarriage.

A comprehensive guide to the amniocentesis test is provided below; it deals with the following areas:

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