The menstrual cycle is typically divided into four main phases:
1. Menstrual Phase:
- Duration: About 3-7 days.
- Description: This is the phase where the lining of the uterus (endometrium) is shed through the vagina, resulting in menstrual bleeding.
2. Follicular Phase:
- Duration: About 11-27 days (varies widely among individuals).
- Description: This phase starts on the first day of menstruation and continues until ovulation. The pituitary gland releases follicle-stimulating hormone (FSH), which stimulates the ovaries to produce follicles. One of these follicles will mature into an egg. Estrogen levels rise, leading to the thickening of the uterine lining in preparation for a potential pregnancy.
3. Ovulation Phase:
- Duration: Approximately 24 hours.
- Description: Ovulation occurs roughly in the middle of the cycle and is triggered by a surge in luteinizing hormone (LH). The mature egg is released from the ovary and is available for fertilization. This is the most fertile period of the cycle.
4. Luteal Phase:
- Duration: About 14 days.
- Description: After ovulation, the ruptured follicle transforms into the corpus luteum, which secretes progesterone. This hormone further prepares the uterine lining for a potential pregnancy. If the egg is not fertilized, the corpus luteum breaks down, leading to a drop in progesterone and estrogen levels, and the cycle restarts with the menstrual phase.
Each phase is regulated by a complex interplay of hormones, primarily estrogen and progesterone, along with FSH and LH from the pituitary gland.
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