Between about 15 and 18 weeks, your physician might recommend that you receive an amniocentesis. This diagnostic procedure is offered to expectant patients who are at risk for various genetic abnormalities, including Down syndrome, or for other disorders that may be detected by amniocentesis.
An amniocentesis is performed to obtain a sample of the amniotic fluid surrounding a baby. To obtain the sample, the physician will insert a long needle through the abdominal wall into a pocket of amniotic fluid in the uterine cavity. From that fluid pocket, a small amount of amniotic fluid is withdrawn through the needle. The fluid is sent to a laboratory for analysis of the chromosomal patterns of cells your baby has shed into the fluid as a part of the baby's growth cycle. Through analysis of the baby's cells found in the fluid, physicians can get a detailed description of your baby's chromosomes. Results from an amniocentesis carry a 99.4 percent accuracy rate.
Patients generally feel very little discomfort during amniocentesis procedures. Physicians often use ultrasound imaging to help guide the needle away from the baby, the umbilical cord, and the center of the placenta. But an amniocentesis is not without risks. In a small number of pregnant women, an amniocentesis can result in complications, including the loss of the pregnancy. Any pregnant woman may request an amniocentesis, but patients considering the procedure should discuss the risks and benefits of the procedure with a physician or genetic counselor. Another highly accurate prenatal chromosomal test is called Chorionic Villus Sampling (CVS). Similar to the amniocentesis, the physician uses ultrasound imaging as a guide while performing the procedure. For a CVS, a thin, plastic tube is inserted into the abdomen or through the vagina in order to collect a small sampling of placental tissue. An advantage to CVS over amniocentesis is that the procedure can be performed earlier in a pregnancy, around 10 weeks. But, CVS does carry a slightly higher risk of complications in comparison to an amniocentesis.
Neither procedure tests for every genetic abnormality.Terms to Know
Amniocentesis: An invasive procedure performed by a physician to collect a sample of amniotic fluid from a developing pregnancy to check for fetal genetic abnormalities. Ideally performed between 15 and 18 weeks.
Chorionic villus sampling (CVS): An invasive procedure performed by a physician to collect a sample of placental tissue from a developing pregnancy to look for fetal genetic abnormalities. Can be performed as early as 10 weeks.
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Images courtesy of the American Institute of Ultrasound in Medicine (AIUM.org).