**Core Concept**
During pregnancy, the cardiovascular system undergoes significant changes to meet the increased metabolic demands of the mother and the developing fetus. Cardiac output, which is the volume of blood pumped by the heart per minute, increases to meet the demands of pregnancy.

**Why the Correct Answer is Right**
Cardiac output increases by approximately 40% during pregnancy due to a combination of factors, including:
* Increased blood volume: Plasma volume increases by about 40% to 50% to meet the demands of the growing fetus and the mother's increased metabolic rate.
* Increased heart rate: The heart rate increases by about 10-20 beats per minute to compensate for the increased blood volume and metabolic demands.
* Increased stroke volume: The stroke volume, which is the amount of blood pumped by the heart with each beat, also increases to meet the demands of pregnancy.
* Decreased systemic vascular resistance: The systemic vascular resistance decreases to allow for increased blood flow to the placenta and the fetus.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because cardiac output does not decrease during pregnancy. In fact, it increases significantly to meet the demands of the growing fetus and the mother's increased metabolic rate.
* **Option B:** This option is incorrect because cardiac output does not remain the same during pregnancy. While the resting cardiac output may be similar to that in non-pregnant women, the cardiac output during exercise or stress is significantly increased during pregnancy.
* **Option C:** This option is incorrect because cardiac output does not increase only during the first trimester of pregnancy. The increase in cardiac output is a gradual process that occurs throughout pregnancy, with the greatest increase occurring during the second and third trimesters.

**Clinical Pearl / High-Yield Fact**
It is essential for healthcare providers to recognize the changes that occur in the cardiovascular system during pregnancy to provide optimal care for pregnant women. A 10-20% increase in cardiac output during pregnancy is a normal adaptation, but significant increases or decreases in cardiac output may indicate underlying conditions that require medical attention.

**Correct Answer: D.**