For prenatal screening of neural tube defects, which biomarker is used?

• A. HCG
• B. Folic acid levels
• C. Prolactin
• D. Alpha-fetoprotein

Answer: (D)

Alpha-fetoprotein is the biomarker used to screen for neural tube defects during pregnancy. It is produced by the fetal liver and yolk sac, and when the neural tube closes incompletely, AFP can leak into the amniotic fluid and cross into the maternal blood. This leads to elevated maternal serum AFP in the midtrimester (typically around 16–18 weeks). An elevated AFP prompts further evaluation with detailed ultrasound and, if indicated, analysis of amniotic fluid for AFP and acetylcholinesterase to confirm open neural tube defects. While other factors can affect AFP levels, the finding of increased AFP specifically points toward the possibility of an open neural tube defect. Folic acid reduces the risk of these defects but is not a screening biomarker itself.