The maternal serum alpha-fetoprotein (MSAFP) test is a standard blood screening performed during the second trimester of pregnancy. It is designed to measure the level of a specific protein in an expectant mother’s blood, which can help assess the chances of certain developmental conditions in the fetus. This test is part of routine prenatal care and is offered to pregnant individuals to gather more information about the pregnancy.
It is important to understand that the MSAFP test is a screening tool, not a diagnostic one. This means it identifies individuals who may have an increased likelihood of specific issues, but it does not confirm a diagnosis. The results are used to determine if more definitive testing might be recommended. Often, this screening is included in a larger panel of blood tests, such as the triple or quad screen, which measures other substances to provide a more comprehensive risk assessment.
What the MSAFP Test Screens For
The MSAFP test measures the amount of alpha-fetoprotein (AFP), a protein that is produced by the fetal liver and yolk sac during development. This protein enters the amniotic fluid and crosses the placenta to circulate in the mother’s bloodstream. The concentration of AFP in the mother’s blood changes in a predictable pattern throughout pregnancy, so measuring it at a specific time can provide insights into fetal development. An unusual level of AFP can be associated with a higher probability of certain conditions.
Significantly elevated AFP levels are most commonly linked to open neural tube defects (NTDs). These are birth defects of the brain and spine that occur when the neural tube fails to close completely during the early weeks of pregnancy. Two of the most noted NTDs are spina bifida, where the spinal column does not form properly, and anencephaly, a condition involving the absence of major portions of the brain and skull. High AFP levels can also point to abdominal wall defects, where the intestines or other organs protrude through an opening in the abdomen, such as in gastroschisis or omphalocele.
Conversely, low levels of AFP can sometimes be associated with certain chromosomal abnormalities like Down syndrome. It is also recognized that factors unrelated to fetal anomalies can cause atypical AFP levels. For instance, an inaccurate estimation of the gestational age, or the presence of a multiple gestation, such as twins or triplets, can lead to results that fall outside the expected range.
The Testing Procedure
The MSAFP test is a straightforward procedure performed during a specific window of pregnancy. Healthcare providers typically offer the test between the 15th and 20th week of gestation, with the ideal timeframe being between 16 and 18 weeks. This timing is important because the normal reference ranges for AFP levels are highly dependent on the precise gestational age of the fetus.
The process itself involves a simple blood draw from a vein in the mother’s arm. A healthcare professional will collect a small sample of blood into a vial, which is then sent to a laboratory for analysis. The procedure is routine, non-invasive, and carries no direct risk to the developing fetus.
Once the blood sample arrives at the laboratory, technicians use a method called immunoassay to measure the concentration of AFP. The results are typically available within one to two weeks, after which a healthcare provider will discuss them with the expectant parent.
Understanding Your MSAFP Results
The raw measurement of alpha-fetoprotein in the blood is converted into a more meaningful value known as the Multiple of the Median (MoM). The MoM is a calculation that compares the individual’s AFP level to the median, or middle, AFP level of other pregnancies at the exact same week of gestation. This approach standardizes the results, accounting for the natural fluctuation of AFP levels as pregnancy progresses. A MoM of 1.0 indicates that the AFP level is exactly average for that stage of pregnancy.
A result is generally considered outside the typical range if the MoM value is significantly higher or lower than the median. For example, a high MoM, often set at 2.0 or 2.5 depending on the laboratory, is associated with an increased chance for a neural tube defect. This is because an opening in the fetal spine or abdomen allows more AFP to leak into the amniotic fluid and subsequently into the maternal bloodstream.
A low MoM value may be linked to a higher probability of certain chromosomal issues, such as Trisomy 21 (Down syndrome). However, it is important to remember that most “out-of-range” results are not due to a fetal anomaly. An abnormal result simply indicates that additional, more conclusive testing should be considered to get a clearer picture.
Follow-Up After an Abnormal Result
The immediate recommendation after an abnormal result is to consult with a healthcare provider or a genetic counselor. These professionals can explain the results in detail, discuss the specific level of risk, and outline the available options for further evaluation. They provide context for the screening result and guide parents through the subsequent steps.
The most common follow-up action is a detailed Level II ultrasound. This specialized ultrasound provides a thorough examination of the fetal anatomy, including the spine, brain, and abdominal wall, to look for physical evidence of a defect. An ultrasound can also confirm the gestational age of the fetus and determine if there is more than one baby, both of which can explain an unusual AFP level. In many cases, the ultrasound provides reassurance that the pregnancy is developing as expected.
If the ultrasound does not resolve the uncertainty or if it identifies a potential concern, amniocentesis may be offered. This diagnostic procedure involves collecting a small sample of amniotic fluid to directly measure AFP levels and analyze the fetal chromosomes. Amniocentesis can definitively diagnose or rule out open neural tube defects and chromosomal abnormalities.