The Four Key Phases of the Menstrual Cycle
A comprehensive guide to how menstruation, the follicular phase, ovulation, and the luteal phase interact each cycle.
Cycle literacy - Cycle Phases & Hormones, about 3 min read
How Your Cycle Operates
The menstrual cycle is a synchronized hormonal feedback loop directed by the brain (hypothalamus and pituitary gland) and the ovaries. A typical cycle is counted from the first day of menstrual bleeding (Cycle Day 1) until the day before the next period starts.
While the classic textbook example describes a 28-day cycle, healthy cycles naturally range anywhere from 21 to 35 days in adults, and up to 45 days in adolescents.
The Four Main Phases
1. Menstruation (Early Follicular): The shedding of the endometrial lining when pregnancy has not occurred. Estrogen and progesterone sit at baseline levels.
2. Follicular Phase: Under follicle-stimulating hormone (FSH), follicles develop in the ovaries. One dominant follicle emerges and pumps out estradiol (estrogen), rebuilding the uterine lining.
3. Ovulation: A surge of luteinizing hormone (LH) triggers the release of a mature ovum into the fallopian tube.
4. Luteal Phase: The empty ovarian follicle transforms into the corpus luteum, producing progesterone to prepare the uterus. If the egg is not fertilized, the corpus luteum dissolves, hormone levels plunge, and menstruation begins anew.
Sources
- American College of Obstetricians and Gynecologists: The Menstrual Cycle: Menstruation, Ovulation, and How Pregnancy Occurs
- World Health Organization: Menstrual health
- American College of Obstetricians and Gynecologists: Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign (Committee Opinion No. 651)