What changes are observed in the uterus if fertilisation does not occur?

Explanation

### Step-by-Step Solution

1. **Understanding the Uterus's Role**: The uterus prepares for a potential pregnancy by thickening its inner lining, known as the endometrium. This thickening occurs in anticipation of the implantation of a fertilized egg.

2. **Absence of Fertilization**: If fertilization does not occur, the body recognizes that there is no embryo to implant in the uterus. As a result, the thickened lining is no longer needed.

3. **Degeneration of the Endometrium**: The thickened endometrial lining begins to break down. This process is a natural response to the absence of a fertilized egg.

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