The menstrual cycle is typically divided into four phases:
1. Menstrual phase: This is the first phase of the menstrual cycle and it marks the beginning of the cycle. It is characterized by the shedding of the endometrial lining of the uterus, which results in menstrual bleeding. The menstrual phase usually lasts between 3-7 days, but can vary from woman to woman.
2. Follicular phase: This phase begins on the first day of menstrual bleeding and lasts for about 14 days. It is characterized by the growth and maturation of ovarian follicles, which are structures that contain developing eggs. During this phase, the levels of estrogen and progesterone increase, which causes the endometrium to thicken.
3. Ovulatory phase: This phase marks the release of a mature egg from the ovary. The surge in luteinizing hormone (LH) triggers ovulation, which usually occurs around day 14 of the menstrual cycle. The egg travels through the fallopian tube towards the uterus, where it may be fertilized by sperm.
4. Luteal phase: This phase begins after ovulation and lasts for about 14 days. It is characterized by the formation of the corpus luteum, which is a temporary endocrine gland that secretes progesterone. If fertilization occurs, the embryo will implant in the endometrium. If fertilization does not occur, the corpus luteum will degenerate and hormone levels will decrease, which will trigger the start of a new menstrual cycle.
The first menstrual episode is called menarche and typically occurs between the ages of 11-14 years old. It may take a few cycles for the menstrual cycle to become regular after menarche.