**Core Concept**
Pregnancy-induced physiological changes involve various endocrine, cardiovascular, and respiratory adaptations to ensure a healthy pregnancy and fetal development. These changes are primarily mediated by hormonal fluctuations, particularly the increase in estrogen and progesterone levels.

**Why the Correct Answer is Right**
During pregnancy, the uterus expands significantly to accommodate the growing fetus. The increase in progesterone levels causes relaxation of the uterine smooth muscle, which helps to prevent premature contractions and supports fetal growth. Additionally, the increased blood volume and cardiac output help to meet the oxygen and nutrient demands of the fetus. The respiratory system also undergoes changes, including an increase in tidal volume and respiratory rate, to compensate for the increased oxygen demand.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the cardiac output actually increases by 30-50% during pregnancy, not decreases. This increase helps to meet the oxygen and nutrient demands of the fetus.
**Option B:** This option is incorrect because the blood volume actually increases by 40-50% during pregnancy, not decreases. This increase helps to provide the necessary oxygen and nutrients to the fetus.
**Option C:** This option is incorrect because the respiratory rate actually increases during pregnancy, not decreases. This increase helps to provide the necessary oxygen to the fetus.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the increased blood volume and cardiac output during pregnancy can lead to a higher risk of cardiovascular complications, such as preeclampsia and cardiac failure.

**Correct Answer:** D.