The Alpha-Fetoprotein (AFP) test is a prenatal screening. This blood test assesses the risk of a developing fetus having certain conditions. It helps expectant parents and healthcare providers understand potential risks early.
Alpha-Fetoprotein Explained
Alpha-fetoprotein (AFP) is a protein primarily produced by the developing fetus. Initially, it is made by the fetal yolk sac and later, predominantly by the fetal liver and, to a lesser extent, the gastrointestinal tract. This protein is a major component in fetal serum during the first trimester, peaking around 10 to 13 weeks of gestation.
A small amount of AFP travels from the fetus into the surrounding amniotic fluid. From there, it crosses the placenta and enters the mother’s bloodstream. The concentration of AFP in maternal blood gradually increases throughout pregnancy, usually rising from about the 14th week until approximately 32 weeks.
Why the Test is Performed
The AFP test functions as a screening tool, not a definitive diagnostic test. Its primary purpose is to identify pregnancies that may have a higher risk for specific birth defects or genetic conditions. This early assessment allows for further investigation if needed.
Elevated AFP levels can indicate an increased risk of neural tube defects (NTDs), conditions where the brain or spinal cord do not form properly. Examples include spina bifida, where the spinal column doesn’t fully close, and anencephaly. The test can also screen for abdominal wall defects, such as omphalocele or gastroschisis.
Conversely, lower-than-expected AFP levels may suggest a higher risk of chromosomal abnormalities, including Down syndrome (Trisomy 21) or Edwards syndrome (Trisomy 18). The AFP test is often performed as part of a multiple marker screen, which combines AFP with other markers for a more comprehensive risk assessment.
How the Test is Conducted
The AFP test is a simple blood test performed on the pregnant mother. A healthcare professional draws a blood sample from a vein, typically in the arm. The sample is then sent to a laboratory for analysis.
This screening is generally conducted between 15 and 20 weeks of gestation. Optimal results are often obtained when the test is performed between 16 and 18 weeks of pregnancy. No special preparation, such as fasting, is required before having the blood drawn.
Understanding Your Results
After your blood sample is analyzed, the AFP levels are usually reported as a “multiple of the median” (MoM). This value compares your AFP level to the average level for your specific week of pregnancy. Normal AFP levels generally indicate a low likelihood of the screened conditions, though a normal result does not guarantee the absence of a problem.
Elevated AFP levels suggest a higher risk of neural tube defects or abdominal wall defects. Other reasons for elevated AFP can include carrying multiple babies, as each fetus produces AFP, or an incorrect estimation of the pregnancy’s due date. In some cases, elevated AFP can also be associated with certain kidney or placental issues.
Lower-than-expected AFP levels can indicate an increased risk for chromosomal abnormalities. Similar to high levels, an inaccurate gestational age can also lead to an AFP result appearing lower than expected. An abnormal result from the AFP test does not mean the fetus definitely has a problem; it simply signals the need for further investigation.
If an AFP test result is abnormal, healthcare providers recommend additional testing. The first step involves a detailed ultrasound to confirm gestational age, check for multiple pregnancies, and look for any visible structural anomalies. If the ultrasound does not provide a clear explanation or if concerns persist, an amniocentesis might be suggested. This procedure involves withdrawing a small amount of amniotic fluid to analyze fetal cells, providing a more definitive diagnosis of chromosomal abnormalities or neural tube defects.