Maternal Serum AFP Screening Deadline
The optimal time for maternal serum AFP screening is between 16 and 18 weeks of gestation, with testing being reliable when performed between 15 and 20.9 weeks gestation. 1, 2
Optimal Timing and Testing Window
Maternal serum alpha-fetoprotein (MSAFP) screening is a critical component of prenatal care, primarily used to identify pregnancies at risk for:
- Open neural tube defects (ONTDs)
- Anencephaly
- Ventral wall defects
The American College of Medical Genetics provides clear guidance on timing:
- Optimal window: 16-18 weeks gestation 1, 2
- Acceptable range: 15-20.9 weeks gestation 1
- Testing outside this window may yield less reliable results
Importance of Accurate Gestational Dating
Accurate gestational dating is crucial for proper interpretation of MSAFP results:
- If gestational age changes by 2 or more weeks after ultrasound examination, test results must be reinterpreted 1
- If the sample was drawn before 15 weeks, a new sample should be obtained with the correct gestational age 1
- Ultrasound dating of pregnancy reduces false-positive rates and increases detection rates of ONTDs 1
Detection Rates and Interpretation
When performed within the optimal window, MSAFP screening can detect:
- 75-90% of open neural tube defects
- 95% of anencephalics
- 85% of ventral wall defects 1
Results are reported as multiples of the median (MoM) and require adjustment for:
- Maternal weight
- Race
- Presence of insulin-dependent diabetes
- Number of fetuses
- Family history of ONTD 1, 2
Follow-up for Abnormal Results
Cut-off levels for ONTD screening are:
- 2.0-2.5 MoM in singleton pregnancies
- 4.0-5.0 MoM in twin pregnancies 1
When MSAFP is elevated, follow-up should include:
- Genetic counseling
- Targeted (level II) ultrasound examination
- Amniocentesis when indicated 1
Integration with Other Screening
MSAFP is often incorporated into multiple marker screening:
- Women who have elected first trimester screening and/or CVS should still be offered MSAFP screening at 16-18 weeks 1
- MSAFP can be part of "triple test" or "quad screen" for aneuploidy detection 1
Clinical Pitfalls to Avoid
- Incorrect gestational dating: The most common cause of misleading results 2
- Testing too early: Before 15 weeks may yield unreliable results 1
- Testing too late: After 20.9 weeks reduces time for follow-up interventions 2
- Failure to adjust for maternal factors: Weight, race, diabetes status affect interpretation 2
- Inadequate patient education: Patients should be fully informed about the screening process and potential implications 1
MSAFP screening should only be undertaken with comprehensive program components including prompt reporting, appropriate counseling, and adequate confirmatory procedures when indicated 1.