**Core Concept**
The obstetrician is trying to assess fetal lung maturity to determine if a preterm delivery is safe. This involves evaluating the development of the fetal lungs, which requires the presence of surfactant, a substance that reduces surface tension in the alveoli, allowing for easy expansion during breathing.

**Why the Correct Answer is Right**
The most commonly used indicator of fetal lung maturity is the presence of phosphatidylcholine, specifically lecithin, which is a key component of surfactant. The lecithin-sphingomyelin (L/S) ratio is a quantitative measure of this, which is obtained by analyzing amniotic fluid. A higher L/S ratio indicates that the fetal lungs are more mature and have more surfactant, making preterm delivery safer.

**Why Each Wrong Option is Incorrect**
**Option A:** Fetal hemoglobin is not directly related to lung maturity, although it can be used as a marker for gestational age.
**Option B:** Glucose levels in amniotic fluid can be used to assess fetal well-being but are not directly related to lung maturity.
**Option C:** Bilirubin levels in amniotic fluid can be used to assess fetal hemolytic disease but are not related to lung maturity.

**Clinical Pearl / High-Yield Fact**
In cases of suspected Rh incompatibility, the L/S ratio can also be used to assess fetal lung maturity, as Rh incompatibility can lead to fetal hemolysis and subsequent lung immaturity.