The nurse practitioner instructs the pregnant adolescent that several diagnostic tests will be performed during prenatal visits. She should return for serum alpha-fetoprotein (AFP) around:

Rationale: A. 8–14 weeks is typically when first-trimester screening (nuchal translucency and serum markers like PAPP-A and free β-hCG) is performed, not AFP. B. 10–15 weeks is not the standard window for serum AFP testing. C. 16–18 weeks’ gestation is the recommended time for maternal serum alpha-fetoprotein (AFP) testing, which is part of the quad screen to assess risk for neural tube defects and certain chromosomal abnormalities. D. 20–24 weeks is beyond the optimal window for AFP screening; levels may no longer provide accurate risk assessment for neural tube defects.