**Core Concept**
Gestational diabetes mellitus (GDM) is a metabolic disorder that occurs during pregnancy, characterized by high blood glucose levels due to insulin resistance and impaired insulin secretion. The diagnosis of GDM is typically made between 24 and 28 weeks of gestation, when the risk of complications is highest.

**Why the Correct Answer is Right**
The American College of Obstetricians and Gynecologists (ACOG) recommends screening for GDM between 24 and 28 weeks of gestation. This timing allows for early detection and management of GDM, which can reduce the risk of maternal and fetal complications. The oral glucose tolerance test (OGTT) is the standard diagnostic test for GDM, which involves measuring blood glucose levels after a 50-gram glucose load.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because GDM is not typically diagnosed before 24 weeks of gestation, as insulin resistance and impaired insulin secretion may not be as pronounced at this stage.
**Option B:** This option is incorrect because GDM is not typically diagnosed after 32 weeks of gestation, as the risk of complications increases with gestational age.
**Option C:** This option is incorrect because GDM is not typically diagnosed at random, but rather through a standardized screening and diagnostic process.

**Clinical Pearl / High-Yield Fact**
The 50-gram glucose load used in the OGTT is equivalent to about 1 can of soda, and patients should fast for at least 8 hours before the test to ensure accurate results.

**Correct Answer:** C.