The Triple Screen and the Quadruple Screen
During your second trimester, you can expect to be offered a triple screen or quadruple screen. The triple screen is a blood test which can help determine the risk of your baby having Down's syndrome, certain neurological defects, and some other kinds of problems. The quadruple screen adds an extra test which may make the test for Down's more accurate.
The screens can be done any time between the 15th and 22nd weeks of pregnancy, but are most accurate during the 16th to 18th weeks.
Tests in the Triple Screen
The triple screen looks for three substances in the blood:
- Alphafetoprotein (AFP)
- Beta-Human Chorionic Gonadotropin (HCG)
The quadruple screen also looks at
AFP is produced by the developing baby's liver and digestive tract. It passes through the placenta and enters the mother's blood stream.
Estriol is produced by the placenta, from substances released by the developing baby's liver and adrenal glands.
HCG is also produced by the placenta.
Problems the Screen Looks For
The three primary problems that the triple screen looks for are Down's syndrome, neural tube defects, and trisomy 18.
Down's syndrome happens when the baby has an extra copy of chromosome number 21. Chromosomes are long strands of DNA that contain vital genetic information. They're sort of like blueprints for the human body. Normally, humans have one set of chromosomes from the mother and one from the father. But sometimes, a sperm or egg has extra copies. A baby can't develop normally with extra chromosomes. If there is an extra copy of chromosome number 21, the baby will have facial features characteristic of Down's syndrome, some degree of mental retardation, and possibly heart defects or other problems.
Neural tube defects happen when the baby's nervous system doesn't form properly. They include spina bifida and anencephaly. Spina bifida can cause a baby to have paralyzed legs or problems controlling his/her bladder or bowels. Anencephaly is a problem with brain development. Babies with anencephaly do not survive.
Trisomy 18 babies have an extra copy of chromosome 18, which causes them to have very severe mental retardation and other birth defects. Babies with trisomy 18 usually die before they are a year old.
A triple screen can also be abnormal if there are problems with abdominal development, kidney abnormalities, or certain other birth defects.
Accuracy of the Test
The triple screen (or quadruple screen) will not tell you for sure if your baby has a problem. It also can't guarantee that the baby will be normal. What it does is help reveal the risk of the baby having certain kinds of genetic problems or birth defects.
This risk depends on age and other factors, so interpretation of the results is different for every woman. If the test is abnormal, the woman will need further testing to find out if something is really wrong.
A normal triple or quadruple screen is reassuring, but it also can't tell for sure if the baby is normal. For example, a quadruple screen will catch Down's syndrome about 75-80% of the time. That means that it's possible to have a normal quadruple screen, but still have a baby with Down's syndrome. A triple screen will catch about 95% of cases of anencephaly, 85% of severe spina bifida cases, and 65% of Down's syndrome cases. These numbers change according to a woman's age and other risk factors, so it's important for each woman to discuss the test with her health care provider.
If Your Test Is Abnormal
The most common follow-up tests are amniocentesis and ultrasound.
Amniocentesis involves inserting a needle into the uterus and taking a sample of the amniotic fluid. It allows doctors to count the baby's chromosomes and see if he or she really has Down's syndrome or Trisomy 18.
Ultrasound uses sound waves to make a picture of the baby. It can reveal many kinds of birth defects.
Other Screening Tests
There are other tests on the market which can also catch some kinds of birth defects. Talk to your health care provider to see if he or she recommends any of these tests.