Ruling Out Birth Defects
The prenatal test known as chorionic villus sampling (CVS) consists of removing a small amount of tissue from the outside of the sac in which the developing fetus resides. The tissue then undergoes testing which can confirm or rule out specific birth defects. This test is performed early in pregnancy, somewhere between 10 and 12 weeks after a woman's last menstrual period.
CVS is a diagnostic test performed when the fetus is at increased risk for chromosomal or genetic birth defects. While amniocentesis can be used to diagnose the very same birth defects, it cannot be performed until 15-20 weeks have passed since the woman's last menstrual period. CVS offers an earlier result. A diagnostic test is different than a screening test. All women receive screening tests for birth defects like Down syndrome, but screening tests cannot provide a confirmed diagnosis for a birth defect.
CVS is not offered to all pregnant women because of the slight risk of miscarriage associated with the test. The test is performed when the first trimester screening tests, performed between 11-13 weeks of pregnancy suggest the fetus is at risk for Down syndrome. Women over the age of 35 are at a greater risk for delivering a child with Down syndrome and have the option of bypassing the screening tests in favor of CVS or amniocentesis, depending on the stage of the pregnancy.
In cases where there was a birth defect in a previous child or pregnancy, women may be offered CVS or amniocentesis. A couple's medical history may suggest that CVS or amniocentesis should be performed in the case where children may be at an increased risk for inherited genetic disorders. This is only done when the condition in question is one that can be diagnosed before birth. Prenatal diagnosis can be accomplished for most chromosomal disorders but the same is not true of genetic disorders.
In CVS, a small sample of chorionic villi, fragments of tissue taken from the section of the placenta that connects to the wall of the uterus, are removed. The cells of this tissue are then analyzed in a lab. The villi tend to have the same biochemical and genetic makeup as the developing fetus.
There are two CVS procedures and the position of the placenta determines which method to choose. The transcervical CVS involves the insertion of a thin tube into the vagina which is then threaded on through to the cervix where it can reach the villi. Gentle suction is applied so that a small sample of tissue is obtained. In transabdominal CVS, ultrasound is used to guide a thin needle into the abdominal wall and on through to the uterus. A syringe is attached to the needle and this is used to remove the villus sample.
After the procedure is performed, your baby's heartbeat will be monitored with ultrasound and you will need to rest for several hours. You may experience some vaginal bleeding or cramping during this time. If there is heavy bleeding, fever, or contractions, you should contact your physician.
Results are available in about a week, though preliminary results may be ready sooner. Some women experience cramping similar to menstrual cramps at the time the sample is taken, but other women say that CVS isn't at all painful. Transcervical CVS is said to feel much like a Pap smear.