**Core Concept:**
Measurement of Maternal Serum Alpha-Fetoprotein (MSAFP) is a screening test for Down syndrome and other chromosomal abnormalities in pregnant women. The test involves analyzing the levels of AFP (Alpha-Fetoprotein) in the mother's blood, as AFP is produced by the developing fetus.

**Why the Correct Answer is Right:**
MSAFP levels should generally be done at 15-18 weeks of gestation (280-320 days). This is because AFP production by the fetus reaches its peak during this period, providing accurate results for screening purposes.

**Why Each Wrong Option is Incorrect:**
A. Prior to 15 weeks: AFP levels are too low, making the test unreliable.
B. After 22 weeks: AFP production starts to decline, leading to less accurate results.
C. A specific gestational age is not mentioned, but the options A and B provide better reasoning.

**Clinical Pearl:**
To optimize the efficiency of MSAFP screening, it is recommended to perform the test between 15-18 weeks of gestation. Early or late testing may lead to inaccurate results.

**Correct Answer:** .