**Core Concept**
Red degeneration in uterine fibroids is a type of ischemic necrosis occurring due to sudden decrease in blood supply, leading to tissue death. This condition is often seen in rapidly growing fibroids, particularly during pregnancy.

**Why the Correct Answer is Right**
The most common time for red degeneration to occur is during pregnancy, especially during the late first trimester or early second trimester. This is because the increased demand for blood and nutrients by the rapidly growing fibroid can lead to ischemia and subsequent necrosis. The rapid growth of the fibroid can outstrip its blood supply, resulting in red degeneration.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because red degeneration is more common in pregnancy, not during the postpartum period.

**Option B:**
This option is incorrect because while red degeneration can occur in any size of fibroid, it is more common in larger fibroids.

**Option C:**
This option is incorrect because while red degeneration can occur in any type of fibroid, it is more common in submucosal fibroids.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that red degeneration is a painless process, and the patient may present with acute abdominal pain, fever, and leukocytosis. A high index of suspicion is required for early diagnosis and management.

**Correct Answer:** D. During pregnancy.