From the Guidelines
The maternal serum alpha-fetoprotein (MSAFP) test should be performed between 15 and 20 weeks of pregnancy, with the optimal time being 16 to 18 weeks gestation. This timing is crucial because AFP levels in maternal blood naturally vary throughout pregnancy, and interpretation of results depends on having accurate gestational age information 1. The test is typically offered as part of standard prenatal screening and is often included in the quad screen or multiple marker screening. No special preparation is needed before the test; it simply requires a blood sample from the mother. MSAFP screening helps identify pregnancies at increased risk for neural tube defects (like spina bifida or anencephaly), certain chromosomal abnormalities (particularly Down syndrome), and other conditions such as abdominal wall defects. Elevated MSAFP levels may indicate neural tube defects, while lower levels might suggest chromosomal abnormalities. It's essential to understand that this is a screening test, not a diagnostic one, so abnormal results require follow-up testing such as ultrasound or amniocentesis to confirm any findings.
Key points to consider:
- The optimal time for MSAFP screening is between 16 and 18 weeks gestation 1.
- Testing can be done between 15 and 20.9 weeks, but the laboratory should be informed of the gestational age at the time the sample was drawn for an accurate interpretation 1.
- Ultrasound dating of the pregnancy reduces the false-positive rate and increases the detection rate of ONTDs 1.
- The test is most accurate if the laboratory is also informed of maternal weight, race, presence of insulin-dependent diabetes, number of fetuses, and family history of ONTD 1.
It's worth noting that while there are various studies supporting the optimal timing of MSAFP screening, the most recent and highest quality study should be prioritized 1. This study provides the most up-to-date guidance on the optimal timing of MSAFP screening, which is between 15 and 20 weeks of pregnancy, with the optimal time being 16 to 18 weeks gestation.
From the Research
When to Perform MSAFP Test
The maternal serum alpha-fetoprotein (MSAFP) test is typically performed between 15 and 20 weeks of gestation [(2,3,4,5)]. This period is considered optimal for screening for neural tube defects and other fetal abnormalities.
Gestational Age Considerations
- The test is strongly gestation-dependent, and accurate gestational age dating is crucial for interpreting MSAFP results 2.
- Women with high MSAFP levels require sonar scan examination to define gestation, exclude twins, and examine the fetus for obvious malformation or death 2.
Screening for Neural Tube Defects
- MSAFP screening during the second trimester has been widely used for detecting neural tube defects 5.
- The combined relative risk estimate for MSAFP screening is 0.25, with a combined protective rate of 75% 5.
- The sensitivity and specificity of MSAFP screening are 75.1% and 97.7%, respectively 5.
Current Recommendations
- While MSAFP measurement has been considered the gold standard screening test for neural tube defects, mid-trimester ultrasonography has gradually replaced its role as a screening method 6.
- Some countries have issued national guidelines to use mid-trimester ultrasonography instead of measuring MSAFP levels as a prenatal screening method for neural tube defects 6.