Menstruation only occurs if the released ovum is not fertilised. Why?

Explanation

### Step-by-Step Solution:

1. **Understanding the Menstrual Cycle**: The menstrual cycle consists of three main phases: the follicular phase, ovulation, and the luteal phase. Each phase plays a crucial role in preparing the female body for potential pregnancy.

2. **Ovulation Phase**: During the ovulation phase, a mature ovum (egg) is released from the ovary. This is a critical moment as it marks the time when fertilization can occur if sperm is present.

3. **Luteal Phase and Corpus Luteum**: After ovulation, the ruptured follicle transforms into a structure called the corpus luteum. The corpus luteum secretes hormones, primarily progesterone, which is essential for maintaining the endometrium (the lining of the uterus).

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