Alpha-fetoprotein (AFP) is a protein produced by a developing fetus. Measuring this protein in the expectant mother’s blood is a common, non-invasive part of routine prenatal care. This screening estimates the risk for certain developmental conditions in the baby. The result acts solely as a screening indicator, suggesting the need for further discussion and potential testing.
Alpha-Fetoprotein: The Basic Biology
Alpha-fetoprotein is the most abundant plasma protein in the human fetus, acting as the fetal counterpart to albumin in adults. It is primarily synthesized early in gestation by the yolk sac and later by the fetal liver and gastrointestinal tract. AFP functions as a carrier protein, transporting molecules such as fatty acids and steroids within the fetal circulation.
As pregnancy progresses, a small amount of this protein crosses the placenta and enters the mother’s bloodstream. The concentration of AFP in the maternal serum steadily increases until it peaks around the third trimester. Because AFP levels change predictably week by week, precise gestational dating is necessary for accurate interpretation of the test result.
The Maternal Serum AFP Screening Test
The Maternal Serum Alpha-Fetoprotein (MSAFP) test is a blood draw typically offered to pregnant individuals during the second trimester. This screening is usually performed between the 15th and 20th weeks of gestation, with optimal timing often cited between 16 and 18 weeks. The procedure involves collecting a venous blood sample from the mother’s arm for laboratory analysis.
This test is often conducted as a component of a larger prenatal screening, such as the quad screen, which measures three or four different substances. Results are calculated by comparing the measured AFP level to the median level expected for that specific week of pregnancy, expressed as a Multiple of the Median (MoM). The final risk assessment is adjusted based on maternal factors, including weight, age, diabetes status, or carrying multiples.
Interpreting High and Low AFP Levels
An abnormal AFP result suggests an increased risk but does not diagnose a fetal condition. When the AFP level is higher than the established cutoff (often 2.5 MoM), it is most commonly associated with open neural tube defects. These conditions, such as spina bifida and anencephaly, involve incomplete closure of the fetal brain or spinal column. Elevated AFP can also indicate abdominal wall defects or an incorrect estimation of gestational age.
Conversely, a lower-than-expected AFP level suggests an increased risk for certain chromosomal conditions, especially when combined with abnormal levels of other serum markers. The most common of these are Down syndrome (Trisomy 21) and Edwards syndrome (Trisomy 18). The test flags pregnancies that warrant a more detailed investigation.
Next Steps Following an Abnormal Screening
If an AFP screening result is outside the normal range, the first step is usually a targeted or level II ultrasound examination. This advanced ultrasound uses high-resolution imaging to confirm gestational age, check for multiple fetuses, and closely examine the fetal anatomy for visible defects. A corrected gestational age may explain the abnormal AFP level, often making further testing unnecessary.
If the ultrasound does not provide a clear explanation for the elevated AFP, other diagnostic procedures may be offered. Amniocentesis is the definitive follow-up for suspected open neural tube defects or chromosomal issues. This procedure involves analyzing a small sample of the amniotic fluid for fetal cells and for the concentration of AFP and acetylcholinesterase (AChE), which are reliable markers for open neural tube defects.