**Core Concept**
During pregnancy, the body undergoes various physiological changes to accommodate the growing fetus and prepare for childbirth. One of these changes is an increase in blood volume, which is essential for meeting the increased oxygen and nutrient demands of the fetus. This increase in blood volume is a critical adaptation that occurs throughout pregnancy.

**Why the Correct Answer is Right**
The correct answer states that total blood volume increases by 40% by term. This is a well-established fact in obstetrics. The increase in blood volume is primarily due to an expansion of the plasma compartment, which accounts for approximately 70% of the increase. This increase in plasma volume is necessary to meet the increased demands of the fetus and to support the growth of the placenta. The increase in blood volume is a gradual process that occurs throughout pregnancy, with the greatest increase occurring in the second and third trimesters.

**Why Each Wrong Option is Incorrect**
**Option B:** The hematocrit, which is the proportion of red blood cells in the blood, actually decreases during pregnancy due to the increase in plasma volume. This decrease in hematocrit is a normal adaptation that occurs to ensure adequate oxygen delivery to the fetus.

**Option C:** Red cell volume does increase throughout pregnancy, but this increase is not limited to 20 weeks. In fact, red cell mass increases by approximately 20-30% by term, which is less than the increase in plasma volume.

**Option D:** The increase in hematocrit during pregnancy is not due to an increase in red blood cell volume relative to plasma volume. Instead, the hematocrit decreases due to the increase in plasma volume, as mentioned earlier.

**Clinical Pearl / High-Yield Fact**
It's essential for healthcare providers to be aware of the changes in blood volume and hematocrit during pregnancy to accurately diagnose and manage conditions such as anemia and hypovolemia.

**✓ Correct Answer: A. Total blood volume increases 40% by term**