Alpha-fetoprotein (AFP) tests in pregnancy
When you are pregnant, substances from your fetus (developing baby) mix with your own blood. One of these substances is alpha-fetoprotein. Alpha-fetoprotein (AFP) is produced by the fetus and can be detected in a blood sample from you. If your doctor recommends an AFP test, it is usually done between weeks 16 and 18 of pregnancy (during the second trimester).
The results of your AFP blood test can alert your doctor to possible birth defects. A high level of AFP can be an indication of a neural tube defect such as spina bifida. It can also indicate chromosomal problems in the developing baby.
However, it’s important to note that high levels of AFP can also be present for other reasons.
- The level of AFP in your bloodstream increases substantially in the fourth to sixth months of pregnancy, so if you have a high result it may indicate that your pregnancy is further advanced than you realised and your due date needs to be calculated again.
- If you are carrying twins your AFP will also be high.
These days, many pregnant women in Australia will have screening tests in the first trimester of pregnancy, which includes an ultrasound scan that will determine the age of the fetus and whether you are pregnant with twins.
Understanding AFP tests
An AFP test is only a screening test - the most it can do is point to a possible problem. If the results of an AFP test point to a problem, other tests will be needed to confirm the results.
In most cases, AFP test results are normal. Even when they are not normal, the results of follow-up tests are most often normal.
When are AFP tests done?
An AFP blood test can be done in the second trimester as one of several tests used to screen for chromosome conditions and neural tube defects.
What do abnormal AFP test results mean?
Most AFP results are negative (normal). This means the test results are within the normal range for your stage of pregnancy.
Sometimes results are positive (abnormal). Often, this is simply because:
- your due date is different than first thought; or
- you are pregnant with twins.
Sometimes, abnormal results mean that the fetus may have one of the following problems:
- neural tube defects (problems with the spine, such as spina bifida);
- defects in the wall of the abdomen; or
- chromosomal defects, such as Down syndrome).
AFP tests may be done together with other blood tests that measure your blood levels of 2 pregnancy hormones. These hormone levels help determine the risk of the baby having Down syndrome.
How accurate are AFP tests?
AFP test results can sometimes be wrong. These are called false negatives or false positives. Be sure to ask your doctor or obstetrician any questions you have about your results.
If your test results are abnormal, you will be offered more tests. These additional tests can confirm whether there really is a problem or whether everything is normal. The tests include the following.
- Ultrasound scan: this uses sound waves to create an image of the fetus.
- Recalculated AFP: a second AFP test, used if ultrasound shows that your due date is different than first thought or that you have twins.
- Amniocentesis: this is a test of the fluid that surrounds the fetus in the womb.
Should I have an AFP test?
Having an AFP test is up to you and, like all screening tests, it is optional. An AFP test can warn you about some fetal birth defects. In Australia, an ultrasound scan at 18-20 weeks if often done instead of AFP tests.
Here are some more facts to help you decide if you want to have the test.
- An AFP test causes no health risk to you or the fetus.
- An AFP test can only point to possible problems with the fetus.
- If the test points to a possible problem, other tests will be needed to confirm the AFP results.
- Even AFP test results that are normal cannot tell for sure that the fetus is healthy.
The 18 to 20 week ultrasound scan can detect abnormalities hinted at by an AFP test.
Other prenatal screening tests
There are several other screening tests that are available to pregnant women to help detect possible birth defects. Check with your doctor about which tests are most suitable for you. These tests may be recommended as an alternative to, or in addition to, AFP tests.
Screening for chromosomal abnormalities
There are tests that can screen for Down syndrome in the first trimester of pregnancy and are more accurate than AFP tests.
A first trimester prenatal test that involves a blood test (performed at 9 to 13 weeks), combined with a special ultrasound scan called a nuchal translucency scan (performed at 11 to 13 weeks) has largely replaced the AFP test as a screening test for Down syndrome. It also screens for several other chromosomal abnormalities.
Cell-free DNA-based screening, also called noninvasive prenatal testing (NIPT), is a newer test that became available in Australia in 2013. This test involves analysing a blood sample for cell-free fetal DNA - small fragments of the developing baby’s DNA that are found in the mother’s bloodstream.
cf-DNA-based screening is very accurate and can be done from 10 weeks to screen for several genetic and chromosomal conditions including Down syndrome. It is sometimes offered as a follow-up test after other screening tests have shown that there is an increased risk of a chromosomal abnormality.
Until recently the testing was carried out overseas, but there are now laboratories in Australia that do this test. However, the test remains expensive and is not available in all areas.
Doctors usually recommend having a 11-13 week ultrasound in addition to NIPT.
Screening for neural tube defects
An ultrasound scan performed at 18-20 weeks (second trimester) can be done instead of, or in addition to, AFP tests to check for neural tube defects. In some cases, an amniocentesis test may be recommended as a follow-up test.
AFP tests and/or ultrasound scan should still be offered to women who have had first trimester screening.
The AFP test is just one of many tests you may have while pregnant. It is certainly not essential. Most women’s AFP test results are normal. Even if they are not, the chances are high that your baby is healthy. Ask your doctor or obstetrician any questions you have about this or any of the other tests that are offered during pregnancy.
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