How the Menstrual Cycle Works

Fertility & Conception
Key Points

  • There are two cycles: ovarian and uterine.
  • The cycles depend on hormones.
  • The hypothalamic–pituitary–gonadal axis refers to hormones working as a feedback loop between the hypothalamus, the pituitary gland and the ovaries.

The Female Cycles

Women are born with all the eggs they will be able to use. Women are born with about 1 to 2 million eggs; by puberty, around 300,000 eggs remain in the ovaries. In each cycle, several eggs start growing inside the follicles, one egg will mature and be released from the ovary (ovulation), and many others will degenerate. Once released, this egg survives up to 24 hours, ready to be fertilised. At the same time, the cervical mucus becomes thinner, allowing the sperm to swim more easily. And the lining of the uterus gets thicker, ready to receive the fertilised egg. If the egg is not fertilised, the uterus lining sheds (menstruation).

The Two Cycles

This process involves twi cycles: the ovarian and uterine, working together in synchrony. The ovarian cycle refers to the development and release of the egg and changes in the follicles. It comprises the follicular phase, ovulation and luteal phase. The uterine cycle describes the preparation of the inner lining of the uterus for implantation and shedding of this lining when implantation has failed. These cycles happen at the same time and depend on each other.

The cycle begins with the first day of bleeding, which is counted as day 1 of Follicular Phase. The cycle ends just before the next menstrual period. Menstrual cycles normally range from about 25 to 36 days. Only 10 to 15% of women have cycles that are exactly 28 days. In at least 20% of women, cycles are irregular. That is, they are longer or shorter than the normal range. Usually, the cycles vary the most and the intervals between periods are longest in the years immediately after menstruation starts (menarche) and before menopause.

The Female Hormones

The reproductive system depends on the several hormones, with levels going up and down at different phases of the cycle. Something you may not be aware of is that the brain plays a key role in the regulation and control of the reproductive system. This is called the hypothalamic–pituitary–gonadal axis. The hypothalamus (located in the central area of the brain), produces gonadotropin-releasing hormone (GnRH) to regulate the production and release of FSH (follicle-stimulating hormone) and LH (luteinizing hormone) in the pituitary gland (located at the base of the brain). FSH and LH are two gonadotropic hormones important for male and female reproduction. The gonads (testicles and ovaries), will produce testosterone and oestrogen respectively.

There is a feedback loop system between the ovaries and the hypothalamus and pituitary gland. Hormone levels vary at every stage of the cycle to allow for follicle growth, ovulation, implantation and menstruation

The Small Book of Fertility Hormones