Prenatal Tests Finally Shifting Focus From Mom to Fetus
For most of the 20th century, prenatal testing focused almost exclusively on the health of the pregnant woman. Fetal health was evaluated only by measurements of the fetal heart rate or rough estimations of the size of the fetus.
The fetus is getting much more attention now, however, because of several new diagnostic tests. Because these tests are relatively new, questions remain about their reliability and appropriate use. All pregnant women should discuss the tests in detail with their physicians before agreeing to undergo them.
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The most commonly performed tests of fetal health are:
Ultrasound imaging: Early in pregnancy, an ultrasound can be used to monitor the growth and development of the embryo, to detect the presence of a multiple pregnancy, and to assess if the embryo is properly located in the uterus. Later in pregnancy, it may also be used to estimate fetal size, to confirm the due date, to identify changes in the uterus that pose a risk to the fetus and to detect certain structural abnormalities in the fetus.
Despite its widespread use, doctors’ routine use of costly ultrasound testing is controversial. Studies have not been able to conclusively demonstrate that it produces significant benefits when performed without a special indication. To date, there are no known harmful effects from ultrasound to the patient or the fetus, but the procedure is not recommended by either the U.S. Preventive Services Task Force or the American College of Obstetrics of Gynecology unless there is a specific reason, such as vaginal bleeding or premature labor.
Alpha-fetoprotein screening: This test measures the amount of a substance called alpha fetoprotein (AFP) in a pregnant woman’s blood. Produced by the fetus during pregnancy, AFP collects in the amniotic fluid that surrounds the fetus and is slowly absorbed into the mother’s bloodstream. Abnormally high levels of AFP in the mother’s blood often indicate an increased risk for fetal abnormalities, such as brain and spinal cord deformities.
Your doctor should offer to perform an AFP test when you are between 16 and 18 weeks pregnant. Keep in mind, however, that the test results can sometimes be misleading. In fact, the AFP level will be normal in 15% of cases where the fetus has a neural tube defect. Also, 90% to 95% of all abnormally high results are “false positives”; that is, although the AFP level is high, the fetus does not have a neural tube defect. For this reason, amniocentesis is generally recommended to confirm the diagnosis if maternal AFP levels are elevated. (In this procedure, a needle is inserted through the wall of the uterus, a small amount of amniotic fluid is removed and the AFP level of the fluid is measured.)
Human chorionic gonadotropin (hCG) and unconjugated estriol measurement: High hCG levels and low levels of unconjugated estriol in a pregnant woman’s blood have each individually been shown to be associated with an increased risk of Down syndrome in the fetus, as have low levels of AFP. Therefore, these three tests are commonly used together to screen for Down syndrome. The test is far from perfect; in about 40% of cases, it will fail to identify fetuses with Down syndrome. Also, in about 5% of the fetuses that test positive for the syndrome, the fetus is actually normal. When these tests are abnormal, the diagnosis should be confirmed with genetic testing.
Genetic screening: Genetic testing uses a tiny sample of cells from the fetus or placenta to examine the chromosomes of the fetus for evidence of genetic abnormalities. The fetal cells can be removed for examination either by amniocentesis or by a procedure known as chorionic villus sampling (CVS). With amniocentesis, typically performed about 16 weeks into the pregnancy, a small amount of amniotic fluid is removed to obtain fetal cells that have been shed into the fluid. The chromosomes within these cells are then examined for any evidence of genetic abnormalities. Results of the test are usually available about two weeks after the procedure is performed.
With CVS, placental tissue is analyzed instead. To obtain the tissue for laboratory analysis, a needle is inserted through the wall of the uterus (or sometimes through the cervix) directly into the placenta. CVS is typically performed earlier than amniocentesis (about 11 weeks into the pregnancy), and results often are available within a week.
With CVS, a miscarriage occurs in about 1 of 100 procedures, contrasted with about 1 out of 200 procedures with amniocentesis. Because the risk of Down syndrome increases significantly with age, most experts agree that genetic testing should be offered to all pregnant women older than 35.
Dr. Jonathan Fielding is the director of public health and the health officer for the Los Angeles County Department of Health Services. Valerie Ulene is a board-certified specialist in preventive medicine practicing in Los Angeles. They can be reached by e-mail at ourhealth@dhs.co.la.ca.us. Our Health runs the second and fourth Mondays of each month.
