Chorionic villi are the fingerlike cells on the outside of an embryo that attach to the uterine wall during implantation. They are the means by which blood and nutrients are transferred from the mother’s blood to the developing fetus and eventually grow to form the placenta. In the first trimester of pregnancy, usually between 10 and 13 weeks' gestation, chorionic villi cells can be tested for genetic abnormalities or disorders in a procedure known as chorionic villus sampling, or CVS. This test can help to identify potential problems with the developing baby and involves removing some of the cells from the area where the placenta is attached to the uterus and examining their genetic makeup. A doctor may recommend this test for some women but discourage it for others depending on a number of factors, including family medical history, risk factors associated with the pregnancy, or existing maternal health conditions.
The decision to have genetic testing performed during pregnancy can be a difficult one for many parents, and health professionals usually examine individual cases carefully before making a recommendation. Testing may be encouraged if one or both parents has a family history of birth defects or other genetic disorders such as Down’s syndrome or cystic fibrosis. Although it can’t identify every potential problem with a pregnancy, CVS can provide accurate results earlier than other tests like amniocentesis, which is typically performed around the fifth month. Also, test results will only tell whether an abnormality exists; they cannot identify the severity of a potential problem. In some cases, CVS is also used as a paternity test.
Sampling of the chorionic villi can be performed in a few different ways. In one procedure, a catheter is inserted into the uterus through the cervix, and suction is used to remove a cell sample. Another less common method is to draw the cells through a needle that is inserted through the abdomen into the placenta.
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Although the procedure is generally considered to be safe, CVS can cause miscarriage. This and other possible risks should be discussed with a doctor before testing is performed. Women who have had bleeding during their pregnancy, have a sexually transmitted disease, or are pregnant with twins may be advised to avoid CVS. Those with uterine cysts or other similar health concerns may also be advised against the procedures for sampling chorionic villi.