Amniocentesis and chorionic villus sampling are both forms of testing that can be completed to learn if an unborn baby is at risk of having a specific birth defect or chromosomal disorder. One of the main items that they both search for is Down syndrome. Both are performed fairly early in a pregnancy and pose some risk of miscarriage to the baby.
With amniocentesis, amniotic fluid is tested for certain types of disorders. To take the fluid, doctors will use an ultrasound machine to guide a needle into the amniotic sac through the abdomen. A small portion of the fluid is then removed and sent to a lab for testing.
Chorionic villus sampling is a bit different, but still similar. With this procedure, a small tube or catheter is placed through the vagina and cervix and into the uterus. A needle is then inserted through this tube where tissue samples of the chorionic villus are then taken for testing purposes.
Both of these procedures are very accurate and pose only a small risk to the baby. However, there is an increased chance that a miscarriage will occur after having the procedures completed. For the most part, chorionic villus sampling increases the risks more than amniocentesis does.