**Core Concept**
The concentration of lecithin relative to sphingomyelin is a key indicator of fetal lung maturity. This ratio is crucial in assessing the readiness of a newborn's lungs for extrauterine life, particularly in cases of preterm birth or maternal diabetes.

**Why the Correct Answer is Right**
During fetal development, the lungs undergo a series of morphological and biochemical changes. One of these changes is the increase in the concentration of lecithin, a type of phospholipid, relative to sphingomyelin. Lecithin is a surfactant that reduces the surface tension in the alveoli, allowing for easier lung expansion and preventing the collapse of alveoli during exhalation. As the fetus matures, the concentration of lecithin increases, while the concentration of sphingomyelin decreases. This shift in the lecithin-to-sphingomyelin ratio is a reliable indicator of fetal lung maturity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the lecithin-to-sphingomyelin ratio does not begin to rise during the first week of gestation. Fetal lung development is a complex process that involves multiple stages, and the increase in lecithin concentration is a late-stage event.
**Option B:** This option is incorrect because the lecithin-to-sphingomyelin ratio does not decrease in the second week of gestation. In fact, the ratio begins to increase around 32-34 weeks of gestation.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that a lecithin-to-sphingomyelin ratio of 2:1 or higher is generally considered indicative of fetal lung maturity. This ratio can be used as a predictor of successful outcome in preterm infants.

**Correct Answer: C. Second week of gestation.