The menstrual cycle
The menstrual cycle is a natural, normal process which most females experience, starting from around the age of 11 years (can be earlier or later) and continuing until the onset of menopause around the age of 49-52 years (in the absence of pregnancy and/or hormonal contraception use). 

The cycle typically occurs over a period of 28 days but can range from 22-35 days in length. During the menstrual cycle there is a cyclical fluctuation of sex hormones (primarily oestrogen and progesterone), which are mediated by the hypothalamic-pituitary axis (a system involving the brain and reproductive organs).

Whilst oestrogen and progesterone are the key sex hormones involved in the reproductive system, they are also vital in the regulation of other physiological systems and maintaining all-round health. In particular, oestrogen is a key regulator of bone resorption (the process by which the bones are absorbed and broken down by the body), has a cardio protective role (serving to protect the heart or coronary arteries from injury or disease), is involved in the immune response, supports the neural system and it is even involved in cognitive processes. 

Therefore, continuous oestrogen exposure through the menstrual cycle post-puberty and pre-menopause could also reduce risk of other health conditions such as osteoporosis and cardiovascular disease.

The follicular phase (phases 1 & 2)

The first 14 days of a 28-day cycle are known as the follicular phase (phases 1& 2).

This phase starts with menstruation (phase 1) with day one marking the onset of menstrual bleeding. Oestrogen and progesterone levels are at their lowest during your period (phase 1) and the days leading up to your period (phase 4).

Oestrogen levels increase throughout phases 1 and 2. Once they reach a set point, there is an ensuing dramatic surge in LH (luteinizing hormone), and this in addition to a surge in FSH cause ovulation to occur, which is normally around day 14 of a typical 28-day cycle. Oestrogen levels fall just prior to the LH surge.After ovulation, LH levels rapidly drop off.

The luteal phase (phases 3 & 4)

After ovulation, the second 14 days are known as the luteal phase (phases 3 & 4).

The latter 14 days sees a gradual increase in progesterone, alongside another increase in oestrogen. Both of these hormones are released in response to the formation of a structure called the corpus luteum, which functions to protect a developing embryo if there has been a fertilisation.

If there has not been a fertilisation, progesterone and oestrogen levels reach a peak around day 22, before gradually decreasing, and return to base levels at day 28 (in a typical 28 day cycle). 

The withdrawal or decline in hormones is the primary trigger for the onset of menstrual blood loss. The cycle then begins again at day one with the onset of bleeding (getting your period).

The decrease in hormones triggers an inflammatory response. This is thought to be part of the cause of PMS symptoms.

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The endometrium

Throughout the cycle the lining (endometrium) of the uterus also thickens and thins as it prepares for implantation and later pregnancy. Both oestrogen and progesterone are involved in the thickening and development of the endometrial lining. If fertilisation does not occur then the blood supply to the uterus lining is reduced in the second half of the cycle as it is no longer required, and the lining is eventually shed, resulting in the onset of the period and bleeding.

At FitrWoman we always advocate for an individualised approach to the menstrual cycle as each person will have their own unique experience and go through their own set of symptoms. What we aim to do is help educate people to learn about their own bodies, understand what is happening at different times of their cycle and manage their symptoms in the best way possible.

The FitrWoman app is designed for you to track your cycle and train smarter by providing personalised training and nutritional suggestions tailored to the changing hormone levels throughout your cycle.

Download the FitrWoman app for iOS and Android.

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*Contributions to this article come from the Female Athlete Podcast (Dr Georgie Bruinvels and Dr Jess Piasecki)

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