**Core Concept**
During pregnancy, various physiological changes occur to accommodate the growing fetus and ensure a safe environment for both the mother and the baby. One of these changes involves the cardiovascular system, where cardiac output increases to meet the increased metabolic demands of the mother and the developing fetus.

**Why the Correct Answer is Right**
The correct answer is related to the increase in cardiac output during pregnancy. This is due to the increase in blood volume, which is estimated to be around 40-50% higher than pre-pregnancy levels. The increase in blood volume leads to an increase in cardiac output, which is necessary to meet the increased metabolic demands of the mother and the developing fetus. This is achieved through a combination of an increase in heart rate and an increase in stroke volume.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the change that takes place in pregnancy. While blood pressure does change during pregnancy, it is not the primary change that occurs.

**Option B:** This option is incorrect because it is not a direct result of the physiological changes that occur during pregnancy. While the kidneys do play a role in pregnancy, the primary change that occurs is not related to kidney function.

**Option C:** This option is incorrect because it is a potential complication of pregnancy, but it is not a change that takes place during normal pregnancy.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the increase in cardiac output during pregnancy is a physiological adaptation that helps to ensure a safe environment for both the mother and the baby. This increase in cardiac output can be seen as early as 6-8 weeks into pregnancy and can continue until delivery.

**Correct Answer:** A. Increase in cardiac output.