What is a multiple marker test?
Your pregnancy produces certain substances -- or markers -- in your blood that contain information about your baby. A multiple marker test (also called a triple screen, quad screen, or maternal serum screening), is a blood test that measures three or four of these substances to find out whether your baby may be at risk for certain birth defects.
The markers measured by the test go by complex names: alpha-fetoprotein (AFP for short), a protein produced by your baby's liver, and two hormones produced by the placenta called human chorionic gonadotropin (hCG), and unconjugated estriol (uE3).
Introduced in the early 1980s, the first maternal blood screening measured only AFP levels and was known simply as the AFP test. Since then, researchers have found that combining the AFP reading with hCG and uE3 data more accurately assesses your risk of having a baby with Down syndrome, an abnormality caused by an extra copy of one chromosome.
Another multiple marker test, called a quad screen, measures the first three substances plus a fourth -- inhibin-A. Adding this hormone into the mix appears to make the test even more accurate at revealing pregnancies at risk of Down syndrome. Not all medical centers offer the test, but most labs in the U.S. measure inhibin-A now.
Who should have this test?
A triple screen is typically offered to all pregnant women, regardless of whether their pregnancy is considered low or high risk. Whether you opt to take the test is up to you.
You should consider taking the screen if you are 35 years or older, have a family history of birth defects, or have had a prior pregnancy with a birth defect. The test is also advisable if you have insulin-dependent diabetes or have used medications or drugs that may be harmful during pregnancy. If you have a viral infection or are exposed to high levels of radiation during your pregnancy, you should also consider having the test.
A multiple marker test is most accurate when performed between the 16th and 18th weeks of your pregnancy. But it may be done anywhere from week 15 to week 20, and in rare cases, up to week 22.
How is the test performed?
Blood is drawn from your arm. There may be a little bruising or soreness at the puncture site, but the test poses no danger to you or your baby. Often the same blood sample is used to test for all three markers, and the results are considered, along with other factors -- like your age, weight, race, and whether you have insulin-dependent diabetes -- to determine your risk factors. Your results are usually available within a week.
What does the test do?
Since this is a screening tool, not a diagnostic tool, the multiple marker test does not confirm whether your baby will or will not have a birth defect. But it can give you more information about your chances of carrying a child with specific abnormalities, including the following:
- Down syndrome. Caused by an extra copy of chromosome 21, this condition affects an estimated one in 733 babies. Down syndrome is characterized by mental retardation and distinct facial features, including slanted eyes, low-set ears and a flat face. Children with the disorder often have heart defects and other health problems as well.
- Trisomy 18. Caused by an extra copy of chromosome 18, this condition affects about one in 7,000 babies. It causes severe mental retardation and numerous physical defects, and is usually fatal within the first few years.
- Neural tube defects. The neural tube is the embryonic structure that develops into the brain and spinal cord. In babies with neural tube defects, the brain or spinal cord doesn't form properly and can result in spina bifida (open spine), which can lead to neurological problems, leg paralysis and bladder and bowel impairment. A more severe neural tube disorder, known as anencephaly, is a fatal condition characterized by an underdeveloped brain and skull. Neural tube defects occur in about 1 in 1,000 pregnancies in the United States each year.
- Abdominal defects. Test results can signal a potential risk of rare defects of the abdominal wall, kidney, or bowel.
What does an abnormal test result mean?
Though it may sound alarming, an abnormal test result does not necessarily mean that your baby has a birth defect. About 5 percent of women who do a quad screen have an abnormal result.
In many cases, an abnormal result just means your baby is older or younger than first thought: Since the range of normal levels of the three markers vary with the baby's age, being even a couple of weeks off can make a big difference. An abnormal result can also occur if you're carrying twins or triplets. In addition, if you have diabetes or if you are overweight (particularly if you are African American) you may get an abnormal result.
If none of these situations apply to you, your practitioner might advise you to take the test again, just to double-check the results. If the results are the same, or if you don't repeat the test, your practitioner will probably suggest additional procedures to confirm or rule out any problems. She may also recommend that you see a doctor who specializes in high-risk pregnancies.
Sometimes there is no known reason for an abnormal multiple marker test result, and a diagnostic test, such as amniocentesis or ultrasound, reveals that your baby is developing normally.
An ultrasound, which uses sound waves to take a picture of your baby, can confirm your baby's age and lets your doctor look at the baby's brain, spinal cord, kidneys and heart. This test can also determine how many babies you're carrying.
If the ultrasound can't pinpoint the cause of your abnormal test results, your doctor might suggest you have an advanced ultrasound, amniocentesis, or both. The good news is that amniocentesis, which examines the fluid surrounding your baby, is close to 100 percent accurate in detecting certain genetic disorders. Unfortunately, the procedure carries a small but significant risk of miscarriage. According to some studies, between one in 200 and one in 400 women miscarry following amnio. However, a 2006 study found the rate of miscarriage following amnio to be one in 1,600. You may want to weigh this risk and discuss your options with your partner, practitioner, genetic counselor or spiritual adviser.
Confirming a diagnosis allows you to pursue medical or surgical interventions that may help, such as fetal surgery in the case of spina bifida. You may also want to begin preparing for a special-needs child, locate support services and resources, or decide whether or not to carry your child to term.
Abnormal triple screen levels may also be a sign that you are at risk of certain pregnancy complications, like miscarriage, preeclampsia, premature birth, or placental abruption, in which the placenta peels away from the uterine wall before delivery. Armed with this information, you and your doctor can be alert for warning signs and plan what to do should such a complication arise.
How accurate is the test?
The triple screen findings lead to the prenatal diagnosis of about 95 percent of cases of anencephaly and 75 to 80 percent of severe cases of spina bifida. The triple screen is also effective in leading to a Down syndrome diagnosis in about 70 percent of cases. (That figure jumps to 80 percent if the quadruple screen is used.)
While these numbers are good, they mean that not every case of spina bifida or Down syndrome will be found. Conversely, some women whose results place them at high risk for carrying a child with a birth defect go on to give birth to a perfectly healthy child. Remember, too, there are birth defects that these tests can't find -- many simply can't be discovered by current screening tests.
Fortunately, the multiple marker screening can reassure the vast majority of women that their baby does not appear to have certain serious birth defects.
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