Maternal Serum Alpha-Fetoprotein (MSAFP) screening is a routine blood test offered during pregnancy. This voluntary screening helps assess the risk for certain fetal conditions by measuring a specific protein in the mother’s blood. It serves as an initial step in identifying pregnancies that might benefit from further evaluation.
Purpose of the Screening
The MSAFP screening primarily identifies pregnancies with an increased likelihood of neural tube defects (NTDs) and abdominal wall defects. Neural tube defects are malformations of the central nervous system, affecting the brain or spinal cord, which develop in early pregnancy. Common examples include spina bifida, an incomplete closing of the spinal column, and anencephaly, the absence of a major portion of the brain and skull.
Abdominal wall defects, such as gastroschisis and omphalocele, are also detectable. Gastroschisis involves intestines protruding outside the baby’s body through a hole near the belly button. Omphalocele involves abdominal organs protruding through the belly button, contained within a sac. The test measures alpha-fetoprotein (AFP), a protein produced by the developing baby’s liver, which passes into the mother’s bloodstream.
The Screening Process
The MSAFP screening involves a simple blood draw from the pregnant individual’s arm. This blood sample is sent to a laboratory for analysis of AFP levels. The test is typically performed during the second trimester of pregnancy, between 15 and 22 weeks of gestation, with an optimal window of 16 to 18 weeks.
MSAFP is often included as one component of a broader prenatal blood test panel known as the “quad screen.” Beyond AFP, the quad screen measures three other markers: human chorionic gonadotropin (hCG), unconjugated estriol (uE3), and inhibin A. These additional markers, produced by the placenta or fetus, contribute to a more comprehensive risk assessment when combined with AFP levels.
Interpreting MSAFP Results
MSAFP results are presented as a risk assessment, often expressed as Multiples of the Median (MoM). This value compares an individual’s AFP level to the median AFP level found in pregnancies of the same gestational age, adjusted for factors like maternal weight, race, and presence of diabetes. A median value is 1.0 MoM for each week of gestation.
High MSAFP levels, above 2.0 or 2.5 MoM, can indicate an increased risk for neural tube defects or abdominal wall defects. However, elevated AFP levels are more commonly due to factors such as an incorrect estimation of gestational age or the presence of multiple fetuses. Other causes for high levels can include fetal-to-maternal bleeding or certain placental conditions.
Conversely, low MSAFP levels, particularly when evaluated as part of the full quad screen, can be associated with an increased risk for chromosomal abnormalities, most notably Down syndrome (Trisomy 21). In pregnancies with Trisomy 21, AFP levels are reduced, while hCG levels may be increased. This test is a screening tool, not a diagnostic one; it identifies pregnancies that may have a higher chance of a condition. A screen positive result, whether high or low, does not confirm a problem, and false positives are possible, occurring in up to 10% of results.
Follow-Up to Abnormal Results
When MSAFP screening results are outside the typical range, additional steps are recommended. The first common follow-up procedure is a detailed or high-resolution ultrasound examination. This imaging study allows healthcare providers to confirm gestational age, a frequent reason for abnormal screening results, and to closely examine the baby’s anatomy for any visible defects of the brain, spine, or abdominal wall.
If the ultrasound does not clarify the reason for the abnormal screening result or if a concern remains, diagnostic testing may be offered. One such diagnostic test is amniocentesis, which involves withdrawing a small sample of amniotic fluid from around the baby. This fluid contains fetal cells that can be analyzed for chromosomal abnormalities or elevated AFP levels, providing a definitive diagnosis for many conditions that MSAFP screens for.