**Core Concept:** Alpha-Fetoprotein (AFP) is a protein produced mainly by the liver and yolk sac of an embryo and fetus during pregnancy. It serves as a marker for fetal well-being and can be detected in maternal blood.

**Why the Correct Answer is Right:** AFP levels are highest in the third trimester of pregnancy, particularly during the second half. This is due to the increased production and secretion of AFP by the liver and yolk sac of the developing fetus.

**Why Each Wrong Option is Incorrect:**

A. Early pregnancy: AFP levels are lowest in the first trimester because of maternal liver uptake and clearance.
B. Second trimester: AFP levels peak around the 16th-22nd week, but they are still lower than in the third trimester.
C. First trimester: AFP levels are low due to maternal liver uptake and clearance.
D. First trimester: As mentioned earlier, AFP levels are low due to maternal liver uptake and clearance.

**Clinical Pearl:** AFP levels can be used to monitor fetal well-being and detect potential abnormalities. Elevated AFP levels may indicate conditions like neural tube defects or fetal death.

**Correct Answer:** C. Second trimester: AFP levels peak around the 16th-22nd week, but they are still lower than in the third trimester.