Menstrual cycle

The menstrual cycle is a complex physiological choreography in which a series of hormonal events take place that control:

  • Recruitment and maturation of an egg (follicular phase)
  • Expulsion of the mature follicle in the fallopian tubes (ovulation)
  • The preparation of the uterus to receive the fertilized egg (luteal phase)
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The classic 28-day menstrual cycle stages

By convention, the first day of menstruation coincides with the first day of the cycle.

1 Follicular (or proliferative) phase - day 1 through day 14

In this initial phase, the follicle stimulating hormone (FSH) initially stimulates growth of a certain number of follicles; in a natural cycle, a single dominant follicle is selected to ensure the final maturation of a single egg. At the same time, the estrogen secreted by the follicle regenerates the lining of the uterus (endometrium which became detached during the previous menstruation).

2 Ovulation - day 14

Ovulation usually occurs at approximately 12 to 14 days before the next menstruation. The day of ovulation therefore varies according to the total length of the cycle. Thanks to the luteinizing hormone (LH), a single egg generally reaches maturity in the natural cycle. The dominant follicle ruptures during ovulation, releasing the mature egg. This oocyte  is then drawn toward the fallopian tube. Some women feel pain upon rupture of this follicle. This is referred to as mittleschmerz or painful ovulation.

3 Luteal (or secretory) phase - day 14 through day 28

During this final phase, the follicle that released the egg changes into a corpus luteum and begins to secrete large quantities of progesterone. This hormone prepares the lining of the uterus to receive the fertilized egg.

In the absence of fertilization, the lining of the uterus becomes detached and menstrual bleeding begins. This is the start of a new cycle.


Hormonal and physiological variation