Alpha-fetoprotein (AFP) is a protein produced by the developing fetus during pregnancy. Measuring the level of this protein in the mother’s bloodstream is a common part of prenatal care. This blood test, typically performed in the second trimester, screens for the probability of certain developmental or genetic conditions in the fetus. Results help healthcare providers determine if further diagnostic testing is warranted.
The Role of Alpha-fetoprotein (AFP)
AFP is the most abundant plasma protein in the developing fetus. It is primarily synthesized by the fetal liver and the yolk sac early in development. Within the fetus, AFP functions similarly to albumin, the main protein in adult blood.
This protein acts as a carrier, transporting substances like fatty acids, bilirubin, and certain hormones to fetal cells. It also helps regulate osmotic pressure, the balance of fluids in the blood. As the fetus develops, AFP passes from the fetal circulation into the surrounding amniotic fluid.
A measurable amount of AFP crosses the placental barrier and enters the maternal bloodstream. The concentration of AFP in the mother’s blood increases steadily throughout the second trimester. This predictable rise provides the basis for using maternal serum AFP levels for screening.
Understanding the MSAFP Screening Test
The test used to measure this protein is the Maternal Serum Alpha-fetoprotein (MSAFP) screening. It involves a simple blood draw, most commonly performed between the 15th and 20th weeks of gestation. Accurate dating of the pregnancy is important because AFP levels change significantly with each week.
The optimal time frame for the blood sample is between 16 and 18 weeks of pregnancy. This timing allows for the most precise interpretation against established normal ranges. The MSAFP test is a screening tool; it only indicates an increased risk and cannot provide a definitive diagnosis.
The AFP level is often measured as part of a larger panel of blood tests, such as a triple or quad screen. These multiple marker tests analyze other substances, including human chorionic gonadotropin (hCG), Estriol, and Inhibin-A. Interpreting the AFP result with these other markers enhances the accuracy of the risk assessment.
Interpreting Abnormal AFP Levels
An “abnormal” MSAFP result means the AFP level is significantly higher or lower than the established median for that specific week of pregnancy. Most abnormal screening results do not ultimately lead to a diagnosis of a problem. The most common reason for an abnormal result is an error in calculating the gestational age.
High Levels
Significantly elevated AFP levels are most strongly associated with open Neural Tube Defects (NTDs). These birth defects occur when the neural tube, which forms the brain and spinal cord, does not close completely early in pregnancy. Examples include spina bifida and anencephaly. With an open NTD, exposed fetal tissue allows excessive AFP to leak into the amniotic fluid, leading to higher concentrations in the mother’s blood.
Elevated AFP can also indicate other issues:
- Abdominal wall defects (gastroschisis or omphalocele).
- Carrying multiple fetuses, such as twins.
- Certain placental abnormalities.
Low Levels
Conversely, a significantly lower MSAFP level is primarily associated with an increased risk for specific chromosomal abnormalities. The most common conditions flagged are Down Syndrome (Trisomy 21) and Edwards syndrome (Trisomy 18). These conditions involve an extra copy of a chromosome.
For Down Syndrome risk, the low AFP level is typically seen with high levels of hCG and low levels of Estriol. This specific pattern of markers is used in the risk calculation. Increased maternal weight can also cause AFP levels to appear lower due to a dilution effect.
Next Steps After an Abnormal Screening
An abnormal MSAFP screening result requires further investigation and is not a cause for immediate alarm, as it only indicates elevated risk. The first step is a detailed review of the initial blood test, including re-evaluation of the mother’s weight, race, and diabetic status, which influence the MoM calculation. Precise re-dating of the pregnancy using ultrasound is also performed.
If the result remains abnormal after re-calculation, a specialized Level II or targeted ultrasound is usually the next step. This high-resolution scan confirms gestational age, checks for multiple fetuses, and looks for physical signs of NTDs or abdominal wall defects. A normal Level II ultrasound often provides reassurance and may eliminate the need for more invasive testing.
If the ultrasound is inconclusive, the provider may recommend amniocentesis. This diagnostic procedure involves withdrawing amniotic fluid for analysis. The fluid can be tested directly for AFP (AFAFP) to detect NTDs, and fetal cells can be analyzed for chromosomal abnormalities. Genetic counseling helps parents understand the specific risks and implications of these tests.