**Core Concept**
During pregnancy, various physiological changes occur to accommodate the growing fetus and prepare the mother's body for childbirth. These changes involve the cardiovascular, hematological, and respiratory systems. A key aspect of pregnancy physiology is the increase in maternal blood volume, which is essential for meeting the oxygen and nutrient demands of the fetus.

**Why the Correct Answer is Right**
The correct answer, "Increase in blood viscosity," is incorrect because pregnancy is characterized by a decrease in blood viscosity, not an increase. This is due to the dilutional effect of increased maternal blood volume, which contains more plasma and less red blood cells. As a result, the concentration of red blood cells decreases, leading to a decrease in blood viscosity. This change is crucial for maintaining optimal blood flow and oxygen delivery to the fetus and mother.

**Why Each Wrong Option is Incorrect**
**Option B:** A fall in serum iron concentration is not typically observed in normal pregnancy. In fact, serum iron levels may remain unchanged or even increase slightly due to increased erythropoiesis. However, serum iron-binding capacity (total iron-binding capacity) may increase due to the increased production of transferrin, a protein that binds iron.
**Option C:** An increase in serum iron-binding capacity is a correct observation in pregnancy. As mentioned earlier, the increased production of transferrin leads to an increase in serum iron-binding capacity, which is essential for transporting iron to the fetus and supporting erythropoiesis.
**Option D:** An increase in blood oxygen-carrying capacity is a correct observation in pregnancy. The increase in maternal red blood cell mass and hemoglobin concentration ensures that the mother's blood can carry more oxygen to the fetus and support its growth and development.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that pregnancy is a state of relative polycythemia, with an increase in maternal red blood cell mass and hemoglobin concentration. This adaptation ensures that the mother's blood can carry more oxygen to the fetus, supporting its growth and development.

**✓ Correct Answer: A. Increase in blood viscosity**