Cervical smear post pic

The Menstrual Cycle

If you'd like to support us, check out our awesome products:

What is the menstrual cycle?

The menstrual cycle is a complex series of physiological changes that occur in women of reproductive age on a monthly basis. The end result is the production of an ovum and thickening of the endometrium to allow for implantation, should fertilisation should occur. The menstrual cycle is orchestrated by the endocrine system through the complex interaction of the hypothalamus, pituitary and gonads (hypothalamic-pituitary-gonadal axis). The entire menstrual cycle lasts around 28 days, with the cycle beginning on the first day of menstruation and ovulation occurring at approximately day 14.

You might also be interested in our premium collection of 1,000+ ready-made OSCE Stations, including a range of O&G stations ✨

Hypothalamic-pituitary-gonadal axis (HPG Axis)

1. The hypothalamus secretes GnRH.

2. GnRH travels down to the anterior pituitary gland and binds to receptors on theΒ gland.

3. This promotes the release of LH (luteinizing hormone) andΒ FSH (follicle-stimulating hormone).

4. LH andΒ FSH travel in the bloodstream to the ovaries.

5. When LHΒ andΒ FSH bind to the ovaries they stimulate the production of oestrogenΒ andΒ inhibin:

  • Oestrogen helps to regulate the menstrual cycle and is an essential component in many other physiological processes
  • Inhibin causes inhibition of activin which is usually responsible for stimulating GnRH production
  • FSH also stimulates the development of ovarian follicles, with the follicle most sensitive to FSH becoming the dominant Graafian follicle
  • LH causes the Graafian follicle to change into the corpus luteum, which begins to produce progesterone
  • Progesterone stimulates the endometrium to become receptive to the implantation of a fertilised ovum

6. Increasing levels of oestrogen, progesterone andΒ inhibin have a negative feedback effect on the pituitaryΒ andΒ hypothalamus.

7. This leads to the decreased production of GnRH, LHΒ andΒ FSH.

8. This, in turn, results in decreased production of oestrogen and inhibin.

9. If a woman becomes pregnant, GnRH, FSH and LH remain inhibited, causing menstruation to cease.

Hypothalamic-Pituitary-Gonadal Axis (HPG axis)
Hypothalamic-Pituitary-Gonadal Axis (HPG axis)

Phases of the menstrual cycle

Follicular phase

1. At the beginning of the menstrual cycle, levels of FSH rise causing stimulation of a few ovarian follicles.

2. As follicles mature they compete with each other for dominance.

3. The first follicle that becomes fully mature begins to produce large amounts of oestrogen.

4. Oestrogen inhibits the growth of the other competing follicles.

5. The single follicle that reaches full maturity during this process is referred to as the Graafian follicle (the oocyte develops within this).

6. The Graafian follicle continues to secrete increasing amounts of oestrogen.

7. Increasing amounts of circulating oestrogen results in:

  • endometrial thickening
  • thinning of the cervical mucus to allow easier passage of sperm
  • inhibition of LH production by the pituitary gland

8. As oestrogen levels rise, they eventually surpass a threshold level, at which point they conversely stimulate LH production, resulting in a spike in LH levels around day 12.

9. The high amounts of LH cause the membrane of the GraafianΒ follicle to become thinner.

10. Within 24-48 hours of the LH surge, the follicle ruptures releasing a secondary oocyte.

11. The secondary oocyte quickly matures into an ootid and then into a mature ovum.

12. The mature ovum is then released into the peritoneal space and is taken into the fallopian tube via fimbriae (finger-like projections).

Luteal phase

13. Once ovulation has occurred LH and FSH stimulate the remaining Graafian follicle to develop into the corpus luteum.

14. The corpus luteum then begins to produce the hormone progesterone.

15. Increased levels of progesterone result in:

  • the endometrium becoming receptive to implantation of the blastocyst
  • negative feedback causing decreased LH and FSH (both needed to maintain the corpus luteum)
  • an increase in the woman’s basal body temperature

16. As the levels of FSH and LH fall, the corpus luteum degenerates.

17. Degeneration of the corpus luteum results in loss of progesterone production.

18. The subsequent falling level of progesterone triggers menstruation and the entire cycle begins again.

However, if fertilisation occurs…

19. If an ovum is fertilised it produces hCG which is similar in function to LH.

20. hCG prevents degeneration of the corpus luteum (resulting in the continued production of progesterone).

21. Continued production of progesterone prevents menstruation.

22. The placenta eventually takes over the role of the corpus luteum (from 8 weeks gestation).

Menstrual cycle


The uterine cycle

The uterus has its own cycle which is driven by the cyclical release of hormones by the ovaries which we’ve previously covered. The inside lining of the uterus is known as the endometrium and is composed of two layers:

  • Functional layer: this grows thicker in response to oestrogen and is shed during menstruation
  • Basal layer: this forms the foundation from which the functional layer develops (i.e. it is not shed)

Phases of the uterine cycle

The uterine cycle has threeΒ phases known as the proliferative,Β secretory and menstrual phases.

Proliferative phase

During the proliferative phase, the endometrium is exposed to increasing levels of oestrogen as a result of FSH and LH stimulating its production.

Oestrogen stimulates repair and growth of the functional endometrial layer allowing recovery from the recent menstruation (increasing endometrial thickness, vascularity and the number of secretory glands).

Secretory phase

The secretory phase begins once ovulation hasΒ occurred.

This phase is driven by progesterone produced by the corpus luteum and results in the secretion of various substances by the endometrial glands, making the uterus a more welcoming environment for an embryo to implant.

Menstrual phase

At the end of the luteal phase, the corpusΒ luteumΒ degenerates (if no implantation occurs).

The loss of the corpus luteum results in decreased progesterone production.

The decreasing levels of progesterone cause the spiral arteries in the functional endometrium to contract.

The loss of blood supply causes the functional endometrium to become ischaemic and necrotic.

As a result, the functional endometrium is shed and exits through the vagina as menstruation.


Print Friendly, PDF & Email