**Core Concept**
During pregnancy, there are significant changes in the maternal coagulation and immune systems to accommodate the growing fetus and prepare for childbirth. One of these adaptations involves an increase in plasma fibrinogen levels, which plays a crucial role in blood clotting.

**Why the Correct Answer is Right**
The increase in plasma fibrinogen levels during pregnancy is a physiological adaptation to prepare the mother for potential blood loss during delivery. Fibrinogen is a glycoprotein produced by the liver that is converted into fibrin, which forms the clot in response to bleeding. The elevated fibrinogen levels help to ensure that the mother can form clots effectively in case of bleeding, thereby reducing the risk of hemorrhage. This increase is typically seen in the second and third trimesters of pregnancy.

**Why Each Wrong Option is Incorrect**
**Option B:** Fibrinogen levels are not decreased during pregnancy; in fact, they are increased to prepare for potential blood loss during delivery.
**Option C:** Thyroglobulins are not decreased during pregnancy; thyroid levels may fluctuate, but thyroglobulin levels remain relatively stable.
**Option D:** IgD levels are not markedly increased during pregnancy; IgG and IgM levels may increase slightly, but IgD levels remain relatively unchanged.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the physiological changes during pregnancy can lead to changes in laboratory values, including an increase in plasma fibrinogen levels. This can affect the interpretation of coagulation studies and other laboratory tests.

**✓ Correct Answer: A. Plasma fibrinogen levels are increased**