Just a few years ago, talking openly about the menstrual cycle was seen as taboo. Now, it’s at the centre of scientific and political debate, and about time too.

Historically, scientific and medical research has failed to include women in its studies, creating a gap in the knowledge of how our menstrual cycles affect our health and wellbeing. Slowly (but surely!) a change is occurring thanks to campaigners and scientists flagging the importance of women’s involvement in research, and the collection of data about menstruation.

No matter when you went to school, this new world of menstrual awareness takes some navigating, and may include some concepts that you haven’t come across before.

Here, we’ll go over how hormones interact in a menstruating woman who isn’t on any kind of hormonal contraception, and how these interactions change throughout her cycle. 

What are hormones?

• Hormones are chemicals that direct parts of your body to behave in certain ways. 
• Hormones relating to your cycle and fertility are known as ‘sex hormones’, and these include ones you might have heard of, such as oestrogen, progesterone and testosterone, as well as other less-well-known hormones, such as Follicle Stimulating Hormone and Lutenising Hormone. 
• Each of these hormones is created in different places around the body, including the glands, the ovaries and the uterus, and some may act as a signal to other parts of the body to start creating a different hormone in order to control a process. 

Your menstrual cycle explained

While period trackers have proved invaluable to many women in recent years, ovulation tracking is only just starting to gain popularity, and not only among women who are trying to conceive; understanding our ovulation patterns can actually give us insight into the rest of  our cycle.

Though this process is clearly a cycle, it also has two distinct parts: the phase before ovulation when the uterus is gearing up to ovulate and prepare the lining for a fertilised egg, and the phase after ovulation when the uterus maintains its lining for any fertilised eggs to implant in. 

Follicular phase

The follicular phase takes place before ovulation, and its start is signalled by the beginning of a period. Both progesterone and oestrogen are low at the start of this phase, and increase as it progresses. 

In women, oestrogen is mainly produced in the ovaries, though its production is controlled by the release of other hormones made in the pituitary gland. Estradiol is the main oestrogen responsible for the menstrual cycle, and its levels increase during the first ten days. When estradiol is detected in the lining of the uterus (aka endometrium), the lining is stimulated to grow and thicken. 

Follicular development

A follicle is a tiny bag of fluid within your ovary. Each follicle contains just one egg, which it stores until it’s mature. 

In the 10-20 days before ovulation, Follicular Stimulating Hormone, produced in the pituitary gland (a bean-shaped gland in the base of the brain) has an X-Factor moment and selects only the most advanced follicle to proceed to ovulation³. This chosen follicle sets off a hormonal chain reaction that results in ovulation (unfortunately not a contract with Simon Cowell and a nationwide tour).

Follicular Stimulating Hormone is kept in check by progesterone and oestrogen, and when these two hormones fall during menstruation, it takes its chance and rises, stimulating the most advanced follicle in the ovary to mature⁴. When the follicle reaches a certain size, it starts to release large amounts of an oestrogen called estradiol, which triggers a ‘Luteinising Hormone surge’, which occurs 34-36 hours before ovulation.

Luteinising Hormone is also made in the pituitary gland, and it is this hormone that signals to the follicle to release the egg, resulting in ovulation. Luteinising Hormone then signals to the remains of the follicle, now an empty structure known as the ‘Corpus Luteum’, to release progesterone. 

Ovulatory phase

During ovulation, the follicle breaks open and releases the mature egg into the fallopian tube. A new follicle needs around 120 days to reach maturity, but while many follicles begin this process, usually just one makes it to full maturity and ovulation². 

This egg stays alive for 12-24 hours, and if it doesn’t get fertilised by sperm, it is simply flushed out of the body. We generally ovulate in the middle of our cycle, though this varies between individuals¹.

Luteal phase

After ovulation, progesterone released by the Corpus Luteum works on reducing estradiol, stopping the growth and thickening of the lining of the uterus. Progesterone, however, maintains the thickness of the lining, and its levels remain high until the Corpus Luteum starts to disintegrate.

If the egg is not fertilised, and it does not implant in the lining of the uterus, then the Corpus Luteum breaks down from day 9 after ovulation⁵. This means that levels of progesterone fall, leading to the breakdown of the uterine lining, which – you guessed it – leaves the body as a period. 

At the end of the luteal phase, the lining of the uterus starts to break down, causing a period to start, and this wonderful cycle begins all over again.