**Core Concept**
During pregnancy, maternal blood volume increases significantly to meet the demands of the growing fetus and the mother's own physiological changes. This increase in blood volume is essential for ensuring adequate oxygenation and nutrient delivery to the fetus.

**Why the Correct Answer is Right**
The maternal blood volume increases by approximately 40-50% during pregnancy, with some studies suggesting a higher increase up to 50%. This expansion of blood volume is necessary to accommodate the increased metabolic demands of the fetus, as well as the mother's own increased metabolic rate. The increase in blood volume is also accompanied by a proportional increase in cardiac output, which helps to maintain adequate blood pressure and perfusion of vital organs.

**Why Each Wrong Option is Incorrect**
**Option A:** 5-10% is an underestimation of the actual increase in maternal blood volume during pregnancy.
**Option B:** 15-20% is still too low to account for the significant physiological changes that occur during pregnancy.
**Option D:** 70% is an overestimation of the actual increase in maternal blood volume, which is not supported by clinical data.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the increase in maternal blood volume during pregnancy is not uniform and can vary depending on the gestational age and individual factors. Understanding this physiological change is crucial for managing pregnancy-related complications and ensuring optimal maternal and fetal outcomes.

**✓ Correct Answer: C. 50%**