The menstrual cycle, explained

Written by Monica Karpinski, medically reviewed by Dr Lydia Gittings

If you are healthy, have ovaries, and have reached puberty — defined as being a stage of maturity where your body is able to reproduce — then it means you will experience a process called menstruation.

This is when your body engages in a monthly cycle of preparing your body for pregnancy by releasing an egg and then building up the lining of the womb (endometrium) for it to implant on once it’s fertilised by male sperm. If the egg isn’t fertilised, then the lining of the womb sheds and is expelled out through your vagina. This is called your period, or menstrual bleed. Once you bleed, the cycle starts again.

It’s estimated that between the ages of 12 and 52, a woman will have 480 periods.¹ Despite this being such a natural and common experience, menstruation carries a specific stigma that affects women globally in different ways.

Understanding the menstrual cycle, and the various ways that it affects women’s lives, is essential when looking at the complete picture of your health.

In brief

• Your period marks the start of your menstrual cycle, which is the process of your body preparing for pregnancy
• At the middle of your cycle, your ovaries release an egg, and build up the lining of the womb for it to implant on if it is successfully fertilised by a sperm
• If the egg isn’t fertilised, it is absorbed back into the body and the womb lining sheds and is expelled through your vagina
• This process is driven by a sequence of hormonal changes and interactions, and is broken down into two phases: follicular and luteal
• The average length of a cycle is 28 days, but cycles can be as short as 21 days to as long as 40 days²
• There are various reasons why your period may be longer or shorter, because there are many different factors in our lives that might alter our hormone levels and interfere with the chain of events that leads to menstruation
• Typically, the volume of blood lost during a menstrual cycle is around 30-40ml. A “heavy period” is considered to be a blood loss of 80ml or more³
• You will stop having periods when you reach menopause, which is when you stop ovulating
• There are many other reasons why you may stop having periods permanently or temporarily, relating to hormones, contraception, lifestyle, and other medical conditions you may have

What happens during the menstrual cycle?

Inside a woman’s womb are two ovaries, which store, develop, and release eggs (ovum) which need to be fertilised by sperm for pregnancy to occur. In general, only one egg is released per cycle. The egg travels down the Fallopian tubes and if it is fertilised by sperm, it will implant on the lining of the womb (endometrium) and develop into a baby.

The endometrium thickens in order to prepare for this fertilised egg, but if it never arrives, the egg is reabsorbed into the body and the womb lining is shed. This “period blood” is actually made up of endometrial tissue that has shed in scales or flakes, red blood cells, and other fluid and enzymes that naturally occur from this process.⁴

This process is divided up into two phases: the follicular phase and the luteal phase.

Phase one: follicular phase

This graph is intended as illustrative

• Your cycle begins on the first day of your period
• As the womb lining breaks down, follicle-stimulating hormone (FSH) is produced
• FSH sends signals to the ovaries to mature new follicles, which are responsible for egg development
• Five-seven follicles are recruited to participate in the cycle, as they develop oestrogen levels rise
• Rising oestrogen stimulates gonadotropin-releasing hormone (GnHR) and lutenising hormone (LH), and builds up the lining of the womb
• Dominant follicle is selected and matures. Oestrogen rises even more and causes a surge of LH
• Matured egg is released (ovulation)

Your menstrual cycle begins on the first day that you bleed. As the lining of the womb breaks down, your pituitary gland in the brain works to produce a hormone called follicle-stimulating hormone (FSH), which more or less does what it says on the tin: it sends signals to the ovaries to mature new follicles, which could develop into eggs. The pituitary also stimulates growth of lutenising hormone (LH) which also works to induce follicular growth.⁵

These follicles grow inside your ovaries and compete with each other for dominance in hopes of being recruited and developing into an egg. When FSH levels rise, around five-seven follicles are selected to participate in that menstrual cycle. As the follicles develop, they release oestrogen, which stimulates production of gonadotropin-releasing hormone (GnRH) and in turn, increases production of LH.

As well as likely being responsible for period cramps, prostaglandins may also stimulate the release of the egg by stimulating the smooth muscle within the ovary

The rising levels of oestrogen in the body work to grow the endometrium, to make it a more favourable environment for an egg to implant. It also makes the endometrium more receptive to progesterone, to prepare it for rising levels of progesterone in the next stage.

By around day seven of this phase, one follicle is usually established as dominant. The other follicles who were recruited but were not successful then die (atresia) while the chosen follicle matures into an egg.

This chosen follicle (preovulatory follicle) causes oestrogen levels to rise even more, which then initiates a surge in LH. The beginning of the LH surge is usually around 34-36 hours before ovulation, and also stimulates growth of progesterone.⁶ Prostaglandins, lipid (fat) molecules that play a role in widening blood vessels and causing muscle contractions in the womb,⁷ are increased in response to increased LH and progesterone. As well as likely being responsible for period cramps,⁸ prostaglandins may also stimulate the release of the egg by stimulating the smooth muscle within the ovary.⁹ Higher levels of FSH are also thought to contribute to freeing the egg from the follicle.¹⁰

Some theories suggest that the egg is released due to increasing pressure on the follicle, but there is no evidence to support this.¹¹

Once the egg is released, LH levels fall.

Phase two: luteal phase

This graph is intended as illustrative

• The follicle that released the egg secretes progesterone to prepare the endometrium for the fertilised egg
• Oestrogen levels also rise
• If the egg is not fertilised, oestrogen and progesterone levels start to fall, and the lining of the endometrium breaks down
• This is your period. The cycle starts again on the day that you start bleeding
• Changing hormone levels during this time are thought to be responsible for a collection of physical and emotional symptoms some women experience, known as premenstrual syndrome (PMS)

After ovulation, what’s left of the follicle after it releases the egg (corpus luteum) works to prepare the endometrium for the fertilised egg by secreting progesterone. Oestrogen levels rise in parallel with progesterone, however, the amount progesterone levels increase is relatively higher. Studies have shown that the corpus luteum is actually the site of production for these hormones (sometimes referred to as steroid hormones).¹²

The corpus luteum is sustained by levels of LH and if you don’t get pregnant, it starts to decline towards the end of your cycle. This causes progesterone and oestrogen levels to fall and the lining of the endometrium to break down. This forms your period, causing the cycle to start again.

The luteal phase is associated with premenstrual syndrome (PMS), a collection of symptoms thought to be caused by the spike in progesterone and oestrogen. PMS symptoms should settle by the time you start your bleed.

Around 3-30% of women experience severe PMS symptoms that greatly impact their daily life.¹³ Severe PMS symptoms may be a sign of premenstrual dysphoric disorder (PMDD), a hormone-based mood disorder that follows changes in the menstrual cycle.

Other factors that affect the menstrual cycle

Because the menstrual cycle is spurred by a sequence of changes in hormone levels, when other factors impact our hormone levels this can disrupt the sequence and cause our period to be irregular, or sometimes even not to come at all. For example, some contraceptives work by releasing synthetic hormones into the body which work to alter the menstrual cycle in ways that prevent pregnancy.

Lifestyle and other medical factors can also upset our hormonal balance and have a knock-on effect on our periods.

Weight loss

There is a minimum of body fat required for maintaining ovulatory function, and when energy supplies are scarce, the body will prioritise functions essential for survival over reproduction. The body has signalling pathways that can sense our nutritional status,¹⁴ and a hormone called leptin communicates with the hypothalamus, a small region in our brain, to regulate energy balance and body weight.¹⁵ Leptin is produced by fatty tissue (adipose tissue) and either directly or indirectly activates centres in the hypothalamus to tell us to decrease food intake or increase our energy expenditure.

Sudden weight loss can cause leptin levels to drop, which has a knock-on effect on the menstrual cycle. When the message that we are deprived of food reaches the hypothalamic-pituitary-gonads axis, this inhibits the GnHR signals required to trigger the chain of events that lead to menstruation.

This is why many women with eating disorders may stop getting their periods. If you have no period for three months and are not pregnant, this is known as secondary amenorrhea.

Gaining or losing weight can carry different risks and benefits for different people, and it’s essential to consider this change within the complete context of your own health

It is important to note that the mechanism that triggers this is depriving the body of energy, which sudden weight loss can bring on. Losing or gaining a healthy amount of weight is unlikely to impact your period. In female athletes who have stopped having periods due to excessive exercise (i.e. energy deprivation), their period returns when their energy expenditure is reduced, even if they have not significantly gained weight.¹⁶

Exercise

Athletes or people who exercise excessively are also likely to have lower body fat stores, i.e. lower stores of energy, which can send a message to the brain to divert energy from reproduction to survival. If you exercise a lot, this can also put a lot of stress on your body, which works to increase amounts of cortisol within the body. Cortisol is sometimes called the “the stress hormone”, for its role in how the body responds to stress.

Cortisol acts as a messenger to the hypothalamus and can also disturb the signal of GnRH,¹⁷ which can disrupt the chain of events that lead to menstruation.

Weight gain

While a minimum amount of fat tissue and energy is required to maintain menstrual function, sudden weight gain can also affect the menstrual cycle. Adipose tissue doesn’t just store energy, but is actually also a site of hormone production, including leptin and aromatase, which works to convert androgens (male hormones) within the body to oestrogens.¹⁹ This can change the hormone balance in the body which, in turn, can disrupt the menstrual cycle.

In a 2012 study of 239 women, those who were “overweight” and “obese” – with a BMI of 25 ≤30 and ≥30 – were found to have higher levels of active oestrogen (called E2), along with decreased levels of progesterone, FSH, and LH.²⁰ Both FSH and LH were found to decrease as BMI increased.²¹

However, despite lower levels of FSH and LH, women with greater BMI appeared to have compensatory mechanisms at work to maintain hormone balance. This means that there may be a greater difference between the highest and lowest amounts of different hormone levels throughout their cycle.²²

It is thought that the increased levels of active oestrogen within the body create negative feedback, which lowers mean levels of LH, and in turn lowers levels of progesterone.²³

Gaining or losing weight can carry different risks and benefits for different people, and it’s essential to consider this change within the complete context of your own health. For example, you may gain or lose weight due to stress or another health condition, which could also impact your menstrual cycle.

These effects can interfere with the hormonal interactions that need to happen for you to have a period—and so, your periods can become irregular or heavier, or even stop altogether.²⁴

Stress, anxiety, and depression

As well as being trigged by stress, increased levels of cortisol are also linked to anxiety and depression. In the same way described above, cortisol released during these mental states may disrupt the signal of GnRH coming from the hypothalamus, which can result in a delayed or even a missed period.

When we are in a state of stress, there are increased levels of corticotropin-releasing hormone (CRH), a hormone involved in the stress response cycle, which leads the pituitary to secrete more of a hormone called adrenocorticotrophin, which triggers the release of cortisol into the system.²⁵ This is linked to a reduced GnRH drive.²⁶

Some women report irregular bleeding on the copper coil (IUD), particularly in the first six months after insertion. This should settle over time. Because the coil is non-hormonal, it should not affect the timing of your menstrual cycle

When periods cease due to disrupted GnRH signals from the hypothalamus, this is known as functional hypothalamic amenorrhea (FHA). Disrupted GnRH signals can lead to anovulation – where you don’t ovulate during a cycle – and low levels of oestrogen.²⁷ This then means that the cyclical changes of oestrogen and progesterone are diminished, which negatively affects the cycle that enables the endometrium to build up.²⁸

Depression can also impact our appetite, which can cause sudden weight loss or gain, triggering processes discussed above.

Contraception

There are many different types of contraception available, that will either be hormonal or non-hormonal in nature. This simply describes the mechanism by which they prevent pregnancy: hormonal contraception uses synthetic hormones to induce reactions within the body that work to prevent pregnancy, while non-hormonal contraception works by other means. Many non-hormonal contraception options are called “barrier methods”, because they work by creating a physical barrier between the sperm and womb.

Hormonal contraception involves ingesting synthetic hormones, which works to alter the overall balance of hormones in the body. For example, the combined oral contraceptive pill (COC) works by keeping the levels of oestrogen and progesterone consistent, suppressing ovulation by preventing hormone surges that trigger it. Some women take the COC pill every day for three weeks, followed by a seven-day break. During this break, levels of progesterone drop slightly, but because the endometrium has not had the chance to build up as much as it usually would, the bleed experienced is usually lighter. However, new guidelines²⁹ indicate that there is no health benefit of taking this break, so women can safely use packets back-to-back if they want to skip periods entirely.

Some women report irregular bleeding on the copper coil (IUD), particularly in the first six months after insertion. This should settle over time. Because the coil is non-hormonal, it should not affect the timing of your menstrual cycle.

There are many different things you should consider when choosing the right contraception for you. It’s best to speak with your doctor about which option would be best.

Other conditions

There are a number of medical conditions that can also have a direct and indirect impact on menstruation. Some common conditions are listed below, but this is by no means an exhaustive list: if you are experiencing irregular periods it’s best to speak with your GP who will advise on how best to proceed.

• Polycystic ovary syndrome (PCOS)
• Endometriosis
• Pelvic inflammatory disease
• Uterine polyps or fibroids
• Premature ovarian insufficiency

Irregular periods are also a common symptom of perimenopause, the period of time leading up to menopause, when your ovaries cease to produce oestrogen.

Featured image is three open menstrual pads in a row. The colours of the pads have been changed through editing to give them a pop-art feel. There is no blood on any of the menstrual pads; they are open as if about to be used

Page last updated January 2023
Next update due 2026