**Core Concept**
Diabetes mellitus in pregnancy is a condition where a woman's blood glucose levels become elevated, posing risks to both the mother and the fetus. This condition can be classified into two main types: gestational diabetes mellitus (GDM) and pre-existing diabetes.

**Why the Correct Answer is Right**
Gestational diabetes mellitus (GDM) occurs in 2-5% of pregnancies worldwide, typically in the second or third trimester. It is characterized by insulin resistance and impaired insulin secretion, leading to hyperglycemia. The exact pathophysiology involves the placenta producing human placental lactogen (hPL), which increases insulin resistance in the mother's body. This condition requires close monitoring and management to prevent complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Does not specifically address the unique features of diabetes in pregnancy.
**Option B:** Incorrectly implies that diabetes in pregnancy is a rare condition, when in fact it affects a significant percentage of pregnancies.
**Option C:** Does not provide a clear feature of diabetes in pregnancy, making it an ambiguous choice.

**Clinical Pearl / High-Yield Fact**
The American College of Obstetricians and Gynecologists (ACOG) recommends screening for gestational diabetes mellitus at 24-28 weeks of gestation in all pregnant women, regardless of their risk factors.

**Correct Answer:** C.