The Menstrual Cycle
The menstrual cycle is the recurring physiological process by which the female reproductive system prepares for potential pregnancy. It comprises coordinated cyclical changes within the hypothalamic-pituitary-ovarian axis and the corresponding endometrial response.
It is conventionally divided into the follicular (proliferative) phase, ovulation, and the luteal (secretory) phase. A typical cycle length is approximately twenty-eight days, though with considerable normal variation.
It provides the essential physiological foundation underpinning the full understanding of the numerous menstrual and reproductive conditions addressed throughout this Atlas.
Physiology of the normal cycle
- The follicular phase extends from the onset of menstruation to ovulation. It is characterised by rising follicle-stimulating hormone secretion, driving recruitment and development of a cohort of ovarian follicles.
- A single, dominant follicle is progressively selected and continues to mature, while the remaining recruited follicles undergo atresia.
- The developing dominant follicle produces increasing quantities of oestradiol. This in turn drives proliferation of the endometrium (the proliferative phase of the endometrial cycle) in preparation for potential implantation.
- Ovulation typically occurs around day fourteen of a standard twenty-eight-day cycle. It is triggered by the luteinising hormone surge, itself resulting from the positive feedback effect of sufficiently high, sustained oestradiol levels produced by the mature dominant follicle. This releases the mature oocyte from the ovarian follicle.
- The timing of ovulation varies substantially according to overall cycle length. The luteal phase length remains relatively constant, while the follicular phase length accounts for the great majority of inter-individual and inter-cycle variation in overall cycle duration.
- The luteal phase extends from ovulation to the onset of the next menstrual period. It is characterised by transformation of the ruptured follicle into the corpus luteum, which produces progesterone (together with continued oestradiol production).
- Progesterone drives secretory transformation of the endometrium (the secretory phase of the endometrial cycle) in preparation for potential embryo implantation.
- In the absence of pregnancy, the corpus luteum undergoes programmed regression (luteolysis) after approximately fourteen days. The resulting fall in progesterone and oestradiol triggers endometrial shedding and the onset of menstruation, marking the beginning of the subsequent cycle.
- Where implantation and pregnancy do occur, human chorionic gonadotropin produced by the developing trophoblast maintains corpus luteum function and continued progesterone production. This prevents luteolysis and the associated endometrial shedding that would otherwise occur.
- This is the essential hormonal basis underlying pregnancy maintenance during this early period, before the placenta itself assumes primary responsibility for progesterone production.
