The Alpha-Fetoprotein (AFP) test is a blood screening offered to pregnant people, primarily during the second trimester. Alpha-fetoprotein is a specific protein produced sequentially by the developing fetus’s yolk sac and then its liver. This protein crosses the placenta into the mother’s bloodstream, where its concentration is measured, a process known as Maternal Serum Alpha-Fetoprotein (MSAFP) screening. The test assesses the chance that the developing baby may have certain structural or chromosomal conditions.
What the AFP Test Screens For
The AFP test examines the protein level in the mother’s blood to estimate the risk for two main categories of conditions. High AFP levels are primarily associated with open neural tube defects (ONTDs). These defects occur when the embryonic structure forming the brain and spinal cord does not close completely early in pregnancy.
ONTDs include conditions like anencephaly, where a major part of the brain and skull is absent, and spina bifida, where the spinal cord is exposed. High AFP results from the protein leaking directly from the exposed fetal tissue into the mother’s circulation. Elevated AFP can also be linked to abdominal wall defects, such as gastroschisis and omphalocele, where internal organs protrude outside the body.
Conversely, significantly low AFP levels, often combined with other markers, suggest an increased risk for certain chromosomal abnormalities. The most common examples are Down syndrome (Trisomy 21) and Edwards syndrome (Trisomy 18). The test is highly effective for detecting open neural tube defects, finding between 75% and 90% of affected babies.
How the Screening Test is Performed
The AFP screening is a straightforward blood test requiring a small sample of blood drawn from the mother’s arm and sent for analysis. The test is most accurate when performed between the 15th and 20th weeks of gestation, with the 16th to 18th week being the most common window.
Accurate dating of the pregnancy is essential for interpreting the results because AFP levels change rapidly throughout the second trimester. The AFP test is now frequently integrated into a multiple marker screen, such as the Triple or Quad Screen. This panel includes other placental hormones, like human chorionic gonadotropin (hCG), Estriol, and sometimes Inhibin A.
Measuring these additional markers alongside AFP allows for a more comprehensive risk assessment, especially for chromosomal conditions. The combined screen results are compared against population standards to provide a risk ratio. There are no risks associated with the procedure itself beyond the minor discomfort of a routine blood draw.
Understanding High and Low AFP Levels
AFP laboratory results are standardized using a metric called Multiples of the Median (MoMs). This calculation compares the mother’s AFP level to the median AFP level of all pregnant people at the exact same gestational age. A result is considered abnormal if it falls outside a specific range, often above 2.5 MoM for high risk or below 0.5 MoM for low risk.
The final risk calculation is adjusted using several maternal factors that influence AFP levels. These factors include the mother’s weight, race, and whether she has pre-existing diabetes. Errors in gestational dating are the most frequent cause of an abnormal AFP result when the baby is ultimately healthy, often resulting in a false positive.
If the pregnancy is earlier than estimated, the AFP value will be lower than expected, potentially suggesting a chromosomal issue. Conversely, if the pregnancy is further along, the AFP will be higher, which could falsely suggest a neural tube defect. Carrying multiple fetuses, such as twins, is another common cause of elevated AFP.
Next Steps After an Abnormal Screening
A screen-positive result indicates a need for further investigation, but it does not automatically confirm a problem. The immediate next step is usually a detailed Level II ultrasound examination. This imaging confirms gestational age, checks for multiple fetuses, and visually inspects the baby for structural defects.
Structural defects, such as an open spine defect or anencephaly, would be consistent with a high AFP level. If the ultrasound confirms correct dating and finds a structural defect, the diagnosis may be considered highly likely. If the ultrasound does not explain the abnormal result, a diagnostic procedure may be offered.
The most common definitive diagnostic test is amniocentesis, typically performed around 15 to 20 weeks of gestation. During this procedure, a small amount of amniotic fluid is withdrawn and tested directly for AFP levels (AFAFP). The fluid is also tested for the enzyme acetylcholinesterase, which indicates an open neural tube defect, and used for chromosomal analysis to confirm or rule out conditions like Down syndrome.
Genetic counseling is a standard part of the process due to the complex nature of the results and the decisions regarding invasive follow-up testing. A genetic counselor helps parents understand the specific risks, the accuracy of the screening, and the risks and benefits of diagnostic options. This ensures parents have the necessary information to make an informed choice.