**Core Concept**
During pregnancy, the body undergoes various physiological changes to accommodate the growing fetus, including an increase in maternal weight. This weight gain is primarily due to the accumulation of maternal tissues, including fat, water, and the fetus itself.

**Why the Correct Answer is Right**
The weight gain during pregnancy is largely attributed to the increased caloric intake and storage of energy as glycogen and fat in the maternal body. The placenta plays a crucial role in producing human chorionic gonadotropin (hCG) and human placental lactogen (hPL), which stimulate lipogenesis and gluconeogenesis, leading to increased glucose and lipid storage. Additionally, the expanding uterus and maternal tissues contribute to the overall weight gain.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while hormonal changes play a role in pregnancy, they are not the primary cause of weight gain.
**Option B:** This option is incorrect because the weight gain during pregnancy is not solely due to fluid retention; while some fluid accumulation occurs, it is a minor contributor to the overall weight gain.
**Option C:** This option is incorrect because the weight gain during pregnancy is not solely due to the fetus itself; while the fetus does contribute to the weight gain, the maternal tissues and other factors also play a significant role.

**Clinical Pearl / High-Yield Fact**
A commonly used guideline for weight gain during pregnancy is the Institute of Medicine (IOM) recommendation, which suggests a total weight gain of 25-35 pounds (11-16 kg) for women of normal weight.

**Correct Answer: C. The fetus itself contributes minimally to the total weight gain.**