11th April 2024

Physiology of the menstrual cycle

Several hormones tightly regulate the mechanisms underpinning fertility and pregnancy. Gonadotrophin-releasing hormone (GnRH) from the hypothalamus stimulates the release of luteinising hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary. This begins the follicular phase, where a steady rise in FSH acts on granulosa cell receptors to cause the conversion of androgens into oestrogen and trigger oocyte development. Androgens are produced by theca cells in response to LH. FSH also promotes follicular development until mid-cycle, where a spike in LH causes ovulation. The residual follicle then becomes the corpus luteum, which produces primarily progesterone, oestrogen, and inhibin A. 

Fertilisation of the ovum prevents regression of the corpus luteum. This is due to the production of β-hCG by the trophoblast cells that surround the embryo. β-hCG stimulates the corpus luteum to continue producing hormones which maintain the endometrial lining. However, involution of the corpus luteum occurs in the absence of fertilisation as high β-hCG levels are not sustained. The decrease in progesterone and oestrogen leads to endometrial shedding (menstruation), and the cycle resets with FSH increasing again to develop new follicles.

Image source: Wikimedia Commons (2010). MenstrualCycle2 en, Licensed under a Creative Commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:MenstrualCycle2_en.svg (Accessed: 07 February 2024).

Table 1 Overview of the main hormones involved in the menstrual cycle

References

The European Society of Human Reproduction and Embryology. POI Guideline Development Group.