## Core Concept
The physiological changes in blood pressure during pregnancy are crucial for understanding the cardiovascular adaptations that occur. In a normal pregnancy, there are specific alterations in both systolic and diastolic blood pressure. These changes are primarily due to the effects of progesterone on vascular smooth muscle and the expansion of maternal plasma volume.

## Why the Correct Answer is Right
During pregnancy, there is a decrease in diastolic blood pressure, particularly in the first and second trimesters. This decrease is attributed to the vasodilatory effect of progesterone, which causes a drop in peripheral resistance. The decrease in diastolic blood pressure is more pronounced compared to systolic blood pressure, leading to a widened pulse pressure. By the third trimester, blood pressures often return towards pre-pregnancy levels or may slightly increase. The correct answer reflects this characteristic change in diastolic blood pressure.

## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests an increase in diastolic blood pressure, which is not a typical change observed during a normal pregnancy. An increase in diastolic blood pressure could indicate hypertension, which is a concern in pregnancy but not a standard physiological change.
- **Option B:** This option implies no change in diastolic blood pressure, which contradicts the known physiological decrease in diastolic blood pressure during the early stages of pregnancy.
- **Option C:** Although not directly provided, if this option does not accurately describe a decrease in diastolic blood pressure, it would be incorrect for the same reasons as options A and B.

## Clinical Pearl / High-Yield Fact
A key point to remember is that a decrease in diastolic blood pressure during early pregnancy is a normal physiological adaptation. However, persistent hypertension or a significant increase in blood pressure at any stage of pregnancy can be a sign of pre-eclampsia or other complications, necessitating close monitoring and management.

## Correct Answer: D. Decrease.