What is the main cause of menstruation in the human female?

Explanation

### Step-by-Step Solution:

1. **Understanding Menstruation**: Menstruation, also known as the menstrual cycle, is a monthly process in human females characterized by vaginal bleeding that typically lasts for 4 to 5 days.

2. **Ovum and Fertilization**: Each month, an ovum (egg) is released from the ovaries around the 14th to 15th day of the menstrual cycle. If this ovum is not fertilized by sperm, it will not implant in the uterus.

3. **Luteal Phase**: After ovulation, the body enters the luteal phase. During this phase, the corpus luteum (the structure formed from the follicle that released the ovum) produces high levels of the hormone progesterone.

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