American Medical Association Family Medical Guide - American Medical Association [278]
These blood tests help evaluate the risk that the fetus has a chromosome abnormality such as Down syndrome (see page 955) or a spinal cord or brain abnormality (neural tube defect; see page 398). These tests are not conclusive, however—3 to 5 percent of women have an abnormal result that is later proven false 90 percent of the time in more precise testing. If the AFP or triple screen test results are abnormal, a doctor usually recommends more accurate testing such as amniocentesis (below) to determine if the fetus has a genetic abnormality.
Amniocentesis
Amniocentesis is a prenatal diagnostic test that is performed between the 14th and 18th weeks of pregnancy. In the procedure, a doctor inserts a hollow needle through the pregnant woman’s abdomen into the uterus to withdraw a sample of amniotic fluid, which surrounds the fetus and contains some fetal cells. The doctor uses ultrasound imaging to locate the fetus and determine the best place to insert the needle to avoid harming the fetus, the placenta, or the umbilical cord. The woman’s abdomen is numbed with a local anesthetic before the needle is inserted.
The fetal cells are grown in the laboratory for up to 2 weeks and then examined for chromosome abnormalities such as Down syndrome, some genetic disorders such as cystic fibrosis and sickle cell disease, and neural tube defects.
Amniocentesis is offered to all pregnant women age 35 or older, whose age puts them at increased risk of having a baby with a chromosome abnormality. Women are also offered amniocentesis if they have a child with a severe birth defect, if someone in their family has a genetic disorder, or if the results of an ultrasound (see page 509) or AFP test (see previous page) show an increased risk of a chromosome abnormality. If you are having amniocentesis and you do not want to know the sex of your fetus, let your doctor know before the procedure because the test can determine sex. Amniocentesis carries a small (1 in 200 to 1 in 400) risk of miscarriage.
Amniocentesis
In amniocentesis, a sample of amniotic fluid is withdrawn from the amniotic sac surrounding the fetus. Doctors use ultrasound imaging to position the needle to avoid the fetus, placenta, and umbilical cord. Amniotic fluid contains cells from the fetus, which are examined to diagnose many disorders, including chromosome abnormalities and open neural tube defects.
Chorionic Villus Sampling
Chorionic villus sampling (CVS) is a procedure performed during pregnancy to diagnose chromosome abnormalities such as Down syndrome and some genetic disorders in the fetus. The test is offered to women who are at increased risk of having a child with a genetic abnormality, such as women 35 or older, women who have a child with a genetic disorder, and women who have a family history of genetic abnormalities. CVS can be performed earlier in pregnancy than amniocentesis (see previous page)—between the 10th and 12th weeks.
Chorionic villi are threadlike projections that form part of the developing placenta and contain the same genetic material as cells of the fetus. In CVS, a small sample of tissue is removed from the placenta through a thin tube (catheter). The sample is removed under ultrasound guidance, either through the vagina and cervix using a minor suction procedure or through the abdomen. CVS poses a small (1 in 100) risk of miscarriage.
First Trimester Screening Test
A newer prenatal screening test is available that can detect Down syndrome (see page 955) and another chromosome abnormality called trisomy 18 syndrome during the first trimester by analyzing a blood sample from a pregnant woman and measuring the thickening of the back of the fetus’s neck with ultrasound imaging. A fetus with abnormal thickening of the back of the neck is at increased risk of having a chromosome abnormality. This test is performed between the 10th and 14th weeks of pregnancy. Because the results of the test are not conclusive, however, more accurate testing, such as amniocentesis (see previous page), is recommended for women whose test results indicate