## Core Concept
Gestational Diabetes Mellitus (GDM) is a condition characterized by high blood sugar levels that are first recognized during pregnancy. It is a significant health concern due to its implications on both maternal and fetal health. The risk factors for GDM include obesity, family history of diabetes, history of delivering a large-for-gestational-age baby, and certain ethnicities.

## Why the Correct Answer is Right
The correct answer, **C. Previous history of GDM**, is a well-established risk factor for developing GDM in a subsequent pregnancy. Women who have had GDM in a previous pregnancy are at a significantly increased risk of developing it again in a future pregnancy. This is because the underlying pathophysiological changes that lead to GDM, such as insulin resistance and impaired insulin secretion, may persist or recur.

## Why Each Wrong Option is Incorrect
- **Option A:** While certain **ethnicities** do have a higher risk of developing GDM, this option is too broad and does not specifically denote a high-risk category compared to a previous history of GDM.
- **Option B:** **Family history of diabetes** is indeed a risk factor for GDM, but it is not as strong a predictor as a previous history of GDM.
- **Option D:** **Polycystic ovary syndrome (PCOS)** is associated with insulin resistance and an increased risk of GDM, but similar to family history, it is not as direct or strong a risk factor as a previous history of GDM.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that women with a history of GDM should be screened early in their subsequent pregnancies, ideally before 24 weeks of gestation, and may even require more frequent monitoring due to their high-risk status.

## Correct Answer: C. Previous history of GDM.