**Core Concept**
Diabetes in pregnancy is associated with an increased risk of fetal abnormalities due to hyperglycemia and its effects on fetal development. The most common fetal abnormalities seen in pregnancies complicated by diabetes include neural tube defects, cardiac defects, and limb abnormalities.

**Why the Correct Answer is Right**
The exact mechanism by which hyperglycemia leads to fetal abnormalities is not fully understood, but it is thought to involve the increased glucose levels in the maternal bloodstream, which leads to an increase in glucose levels in the fetal bloodstream. This, in turn, leads to an increase in insulin levels in the fetus, which can cause an increase in the production of fatty acids and an increase in the activity of enzymes involved in lipid synthesis.

As a result, the fetus may experience an increase in the production of lipids and an increase in the activity of enzymes involved in lipid synthesis, leading to an increase in the production of lipids that can lead to the formation of abnormal lipid deposits in the fetus. This can lead to a range of fetal abnormalities, including neural tube defects, cardiac defects, and limb abnormalities.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the topic of fetal abnormalities in pregnancy with diabetes.
**Option B:** While fetal macrosomia is a common complication of pregnancy with diabetes, it is not a fetal abnormality.
**Option C:** While cardiac defects are a common fetal abnormality in pregnancies complicated by diabetes, they are not the only abnormality.

**Clinical Pearl / High-Yield Fact**
Fetal abnormalities in pregnancies complicated by diabetes are more common than in the general population, and the risk of these abnormalities increases with the severity of the maternal diabetes.

**Correct Answer: C. Cardiac defects.**