Fetal tests: Which ones are right for you?
August 9, 1999
Web posted at: 1:39 PM EDT (1739 GMT)
By John Sussman, M.D., author
(WebMD) -- Genetic testing is a hot topic these days. In fact, most standard fetal tests actually pick up disorders that are at least partially caused by genetics.
These tests can offer you important information about your baby's health. One thing to consider, however, is that the more information you have, the more decisions you're faced with. And although these tests can determine whether you or your partner carries genes that are associated with serious conditions -- such as sickle cell anemia, cystic fibrosis and Down's syndrome -- most of these tests also carry risks.
The types of tests you should have depend upon a number of factors, including your age, past pregnancy history and family history. Your doctor -- and perhaps a genetic counselor -- can help you determine which tests are advisable.
Alpha fetoprotein (AFP)
The alpha fetoprotein (AFP) blood test, done between 15 and 18 weeks, will probably be the first blood test -- other than routine prenatal blood work -- that you get. Also known as the triple, or quad, screen, AFP can detect your risk for having a baby with a neural-tube defect (such as spina bifida, a developmental problem of the spine) or anencephaly (a failure of brain tissue to develop, which leads to death soon after birth).
Detecting the risk of Down's syndrome with AFP
The AFP test can detect which women may be at a higher risk of giving birth to a baby with Down's syndrome, a chromosomal disorder that causes mental retardation and physical irregularities. Traditionally, women over 35 have been the only ones counseled about the risks of chromosomal abnormalities, since their risk for having a Down's syndrome baby is greater than for women under 35. However, the test was actually designed to detect Down's syndrome in women under 35, as the majority of Down's syndrome babies are born to women under the age of 35 (women under 35 as a whole have more babies than those over 35). The AFP test allows these younger women to determine if the risk they face is typical of women their own age (which is relatively low) or more typical of older women (which is considerably higher). If a 23-year-old discovers that her risk of giving birth to a Down's syndrome baby is high, she may decide to have additional tests done to determine whether or not her baby is affected.
The risks of AFP
The problem with AFP is that it is only a screening test, not a diagnostic test: It cannot do anything more than indicate whether you are at a higher-than-normal risk for having a baby with significant health problems. The fact is that the vast majority of women with a so-called "abnormal" AFP will have perfectly normal babies. The anxiety caused to a woman while she waits for further tests to provide more information can be substantial.
Nevertheless, the AFP is an excellent screening test that can lead to diagnosis of potentially severe abnormalities early enough in a woman's pregnancy so that a couple has the chance to consider their options, including termination of the pregnancy, adoption or preparing for the arrival of a special-needs baby.
Chorionic villi sampling (CVS)
Chorionic villi sampling (CVS) is a test done between 10 and 12 weeks -- earlier than most other tests -- to diagnose a variety of genetic and chromosomal abnormalities, including Down's syndrome. During a CVS a doctor will pass either a catheter through the cervix or a needle through the lower abdomen into the uterus to obtain a sample of placental tissue. The advantages of this test are that it can be done in the first trimester, giving a couple more time to consider their options, including first-trimester abortion. The disadvantages are that CVS can be uncomfortable and carries with it a risk of miscarriage, which is about 1 in 100. In addition, CVS cannot diagnose neural-tube defects.
Women who want the earliest-possible diagnosis of certain genetic conditions (most of which can be diagnosed several weeks later with an amniocentesis) may want to consider CVS.
Amniocentesis: The most common invasive diagnostic test
Amniocentesis, which is also an invasive test, is generally done between 15 and 18 weeks of pregnancy, although it can be done as late as 21 weeks. It can diagnose genetic and chromosomal abnormalities as well as certain neural-tube defects, which CVS cannot detect. Amniocentesis can also tell you whether you're having a boy or a girl and, when performed in the third trimester, how developed your baby's lungs are.
What happens during amniocentesis? Amniocentesis involves passing a needle through the lower abdomen into the uterus and withdrawing a small amount of amniotic fluid. This fluid contains cells and other substances from the baby that can lead to diagnosis of all of the conditions mentioned previously, as well as many other genetic disorders for which a particular couple may be at risk. The advantages of this test are that it is accurate, safer than CVS (the miscarriage rate is 1 in 200 to 1 in 300) and can be conducted early enough in pregnancy to allow time for a couple to consider all her options.
Ultrasound: A safe, noninvasive diagnostic test
Ultrasound can be done at any stage of pregnancy to get a look at the baby, placenta, amniotic fluid, uterus and ovaries. It is a noninvasive technique that uses sound waves emitted from a device applied to a mother's abdomen or inserted through the vagina. Ultrasound can be helpful in diagnosing problems in the event of vaginal bleeding or suspicion of twin or tubal pregnancies (when a fertilized egg attaches itself to the fallopian tube instead of the uterus). It can also allow a doctor to assess a baby's well being in the case of a high-risk pregnancy. Though it also can be useful in screening for Down's syndrome and other chromosomal abnormalities, only CVS and amniocentesis can actually diagnosis these conditions.
John Sussman, M.D., is the co-author of "The Unofficial Guide to Having a Baby" (Macmillan, 1999).
Copyright 1999 by WebMD, Inc. All rights reserved.
RELATEDS AT :
Use of sonography during pregnancy
Identifying birth defects
March of Dimes: Frequently Asked Questions
Center for Human and Molecular Genetics
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