|
Why would I need this test?
The alpha-fetoprotein (AFP) test is a prenatal blood test. AFP is a substance produced by the liver of the fetus. Some of this protein is excreted into the amniotic fluid surrounding the fetus and a small amount passes into the mother's bloodstream. The AFP test is a part of what is called the triple test, which looks at three substances normally present in the bloodstream of pregnant women: AFP, and two pregnancy hormones called estriol and human chorionic gonadotropin (HCG). A quad test that adds inhibin to the other three is now available. When AFP is at a high level, the fetus often has a risk of neural tube defects, which occur when the spinal cord or brain does not form completely, as in spina bifida. When combined with measurements of estriol and HCG, it can be used to screen for a high risk of Down's syndrome, a chromosomal disorder that results in mental retardation and physical abnormalities.
How is this test performed?
The AFP and the triple screen are usually done between 16 and 18 weeks after the last menstrual period. A blood sample is taken from a vein in your arm. Results are typically available within one week. Talk with your doctor about your individual risks and concerns regarding AFP testing before consenting to this blood test.
What does an abnormal test result mean?
These tests cannot diagnose a birth defect, but they can indicate when there is a high risk. An increased level of AFP may be found in the blood of a woman whose fetus has a neural tube defect. An AFP level that is lower than normal may be linked to an increased risk of Down's syndrome. Abnormal levels of HCG and estriol may also indicate chromosome abnormalities. An abnormal test result simply means that additional testing is needed. Out of every 1,000 women who take a triple screen test, as many as 100 will have an abnormal result. However, only two or three of those 100 women will have a fetus with a birth defect. For most of the rest, the abnormal result simply indicates the fetus is either a few weeks older or younger than originally thought. The range of normal results varies with the weeks of pregnancy, which is why it is important to know as close as possible the accurate gestational age of the fetus.
What tests are recommended following an abnormal result?
The next step is usually to have an ultrasound examination. Ultrasound, which uses sound waves to take a picture of the fetus, can detect some types of serious birth defects. If the ultrasound doesn't provide an explanation for an abnormal test result, amniocentesis can be done to check for Down's syndrome and other chromosomal abnormalities. In this test, the doctor inserts a thin needle through the abdominal wall and into the uterus to withdraw a few teaspoons of amniotic fluid, which can be studied to see if the fetus's chromosomes are normal. Chorionic villus sampling (CVS) is another test that can detect Down's. The American College of Obstetricians and Gynecologists now recommends that all pregnant women be screened for Down's syndrome - regardless of their age - before the 20th week of pregnancy.
Other tests
Nuchal translucency scan. This test measures the amount of fluid in the back of the baby's neck. This is done in the first trimester at 11 to 14 weeks. A calculation is done using the fluid measurement, the mother's age and the presence or absence of the baby's nasal bone to determine if there are any chromosomal abnormalities. The test also confirms the pregnancy, how far along you are and how many babies you are expecting.
Quad screen. This measures the levels of AFP, HCG, estriol, and inhibin A in your blood - proteins and hormones that only babies make. They pass through the placenta into the mother's blood stream. These levels are tested to see if they are normal, high or low. Many factors, including your weight, race, age and these labs help calculate whether or not the baby may have a chromosomal abnormality or neural tube defect. However, the results are not 100 percent accurate.
|
