**Core Concept**
Gestational diabetes mellitus (GDM) is a condition characterized by high blood glucose levels during pregnancy, which can have various effects on the fetus. The underlying pathophysiology involves insulin resistance and impaired insulin secretion, leading to hyperglycemia. This condition can result in fetal macrosomia, hypoglycemia, and other metabolic and structural abnormalities.

**Why the Correct Answer is Right**
Fetal macrosomia is a common complication of GDM, resulting from excessive fetal glucose uptake and subsequent insulin resistance. This can lead to an increased birth weight, often above the 90th percentile. In contrast, hypoglycemia in the newborn is a result of the abrupt cessation of maternal glucose supply after birth, causing an insulin surge and subsequent glucose depletion.

**Why Each Wrong Option is Incorrect**
**Option A:** Fetal macrosomia is a well-documented effect of GDM, making this option incorrect.
**Option B:** Hypoglycemia in the newborn is indeed a consequence of GDM, as the fetus adapts to the maternal hyperglycemia by producing excess insulin, leading to hypoglycemia after birth.
**Option C:** Respiratory distress syndrome (RDS) is not a direct effect of GDM, although preterm birth, which is a risk factor for GDM, can increase the likelihood of RDS.

**Clinical Pearl / High-Yield Fact**
It's essential to recognize that GDM can lead to neonatal hypoglycemia, which may not be immediately apparent at birth. Monitoring for hypoglycemia in the newborn of a mother with GDM is crucial, especially in the first few hours after delivery.

**Correct Answer:** C. Respiratory distress syndrome