How does the menstrual cycle work?

So, one of the most important factors when trying to conceive is a women’s menstrual cycle. However, very few of us have a detailed understanding of how it works or in particular the patterns of our own cycle in detail. This article hopes to help you understand the importance of you or your partner’s menstrual cycle in your trying to conceive (TTC) journey and get you thinking about whether it is worth tracking your cycle.

What is a normal menstrual cycle?

A normal menstrual cycle is the process of a women’s body releasing an egg and preparing for conception each month (approximately). It involves several hormones: follicular stimulating hormone (FSH), oestrogen, progesterone and luteinising hormone (LH).

Before we start, I’m going to note that 28 days is often quoted as the typical length of a menstrual cycle (only about 10% of women have a menstrual cycle which is 28 days) and you will notice in the below graphic, we use a 28-day cycle to demonstrate how each phase works. However, it is very important to note that every woman is different and the NHS state that a normal cycle for a woman is anything between 21 to 40 days. Thus, the length of each stage and the point of ovulation will vary between different women and even different cycles for the same woman.

There are three phases of a menstrual cycle:

  1. Follicular phase – This phase is from the first day of your period (cycle day 1) to ovulation. Low oestrogen and progesterone cause the shedding of the uterine lining and your period. FSH causes the growth of follicles in preparation for ovulation.
  2. Ovulation – A spike in LH and FSH causes the release of an egg into the fallopian tubes.
  3. Luteal phase – Progesterone and oestrogen rises and thickens the endometrial lining in preparation for implantation of an embryo. If a sperm meets an egg during the luteal phase and fertilises it then that embryo implants, then you become pregnant. If this doesn’t happen then your egg is reabsorbed by the body and the drops in progesterone and estrogen cause the endometrium to shed and you are back to the beginning of the menstrual cycle.

This is a quick overview of a menstrual cycle but let’s look at each stage in a little more detail.

Follicular phase of the menstrual cycle

The follicular phase is usually lasting the first 11-27 days of your cycle. Oestrogen and progesterone are at a low point on cycle day one which causes the start of your period (menses) as the uterine lining sheds having not had an embryo implant in your previous cycle. The low oestrogen and progesterone stimulate the pituitary gland to secrete FSH. Rising FSH stimulates the growth of follicles in the ovaries, usually about 3-30. Follicles are fluid-filled sacs which contain an egg. Several will grow over the first week or so of the follicular phase. One follicle, however, will become dominant usually overtake the others in growth (I like to call this the queen follicle). This follicle will produce oestrogen which then inhibits FSH production and causes the smaller non-dominant follicles to die.


As oestrogen rises it peaks to a level which causes a surge in FSH and LH and the release of the egg in the queen follicle. This typically happens between 10-12 hours after the peak of the LH surge. This surge in LH is what is picked up in ovulation predictor kit (OPK) strips. The egg can only be fertilised in the 12 hours after it is released. This means that you are most fertile in the 3 days before ovulation. The follicle which the egg was released from then closes and become the corpus luteum which as well as producing oestrogen now also produce progesterone.

Luteal phase

The luteal phase usually lasts between 10-17 days. As progesterone increases the endometrial lining continues to thicken and the blood supply to this area also increases in preparation for a fertilised egg (embryo) to implant. It also causes the thickening of cervical mucus to try and prevent sperm or bacteria from entering the uterus and a rise in temperature (this rise in temperature can help us estimate if ovulation has occurred – see ovulation tracking).

Luteal phase if egg fertilises

If the sperm and egg meet at the right time as the egg travels down the fallopian tube and the egg fertilises, then the embryo will travel to the uterus and then hopefully implant in the endometrial lining which has been getting ready to accept the embryo. If this occurs, then you are pregnant. If this happens then the corpus luteum continues to produce progesterone for the following 10 weeks of the pregnancy (until 12 weeks pregnant).

Luteal phase if the egg doesn’t fertilise

If the egg doesn’t fertilise for whatever reason and does not implant, then the corpus luteum disintegrates and progesterone and oestrogen levels drop causing the reabsorption of the egg and the endometrial lining starts to shed which is the start of your period and you are back to the start of the menstrual cycle.

What is a normal period (menses) or menstrual cycle?

Every women’s period is different and what should be considered normal is what is normal for you. However, I will try and describe what most medical professionals would describe as normal for a women’s period.

Timing and Length – Most women have periods lasting 2-7 days, although 5 days seems most common and occur every 21-40 days. Adolescent girls who have just started their menstrual cycles or women in the perimenopause (up to about 8 years before periods stop) are likely to have longer cycles which are more irregular.

Flow – In an average women’s period, women tend to lose up to about 16 teaspoons of blood (up to about 80mls). The most bleeding tends to happen in the first couple days, blood at different points of your period may be brown, pink or bright red but should be red during your heaviest days. Some small blood clots are also normal.

Symptoms – Many women experience a variety of symptoms which could include but is not limited to mild to moderate cramps, back pain, constipation or diarrhoea, acne, bloating, mood swings, hunger, sleep issues and tender breasts.

What is an irregular period or menstrual cycle?

You may be considered to have an irregular period or menstrual cycle if your periods are not 21-40 days apart most of the time or if your periods last less than 2 days or more than 7 days at a time. Your periods are also considered irregular if you experience heavy bleeding or your periods are excessively painful.

Your period would be considered to be heavy if you need to change your sanitary products every hour or two, you need to use two sanitary products together (e.g. using a tampon and pad or menstrual cup and pad at the same time)  or you bleed through your clothes or bedding. Some blood clots are also normal, but they shouldn’t be bigger than 10p piece (or the size of a quarter in the US).

Periods tend to be considered excessively painful if they aren’t easily managed with over the counter medication such as paracetamol (Tylenol), ibuprofen or aspirin or other pain-relieving methods such as heat or TENs (transcutaneous electronic nerve stimulation). It would also be considered excessively painful if it affects your everyday life (e.g. you need to miss school, work or social events due to pain).

If any of the above applies to you or if your periods or menstrual cycle change suddenly you should seek advice from your General Practitioner (G.P) or physician.

Why might you have an irregular period or menstrual cycle?

There are a variety of reasons why irregular periods may occur, here are some possibilities:

  • Uncontrolled diabetes – Uncontrolled blood sugar levels can cause fluctuations in hormone levels which can cause disruptions to a women’s menstrual cycle.
  • PCOS – Women with PCOS may have many hormones out of balance which can cause irregular periods due to irregular ovulation or anovulation (not ovulating). You can see more in our PCOS signs and symptoms article.
  • Eating disorders – Eating disorders such as anorexia and bulimia may mean a woman is not producing enough circulating hormones which may cause an irregular cycle or for periods to stop entirely.
  • Hyperprolactinaemia – Women with an overproduction of the hormone prolactin which affects the production of oestrogen may have irregular periods.
  • Endometriosis – Women with excessively painful periods or an irregular cycle may have endometriosis which is where the endometrial cells shed into areas of the abdomen, they should not be in. You can see a brief overview of endometriosis in our infertility diagnoses
  • Medications – Some medication can give you irregular menstrual cycles such as antipsychotics or anti-epileptic.
  • Premature ovarian insufficiency – Some ladies will lose the function of their ovaries earlier than expected which is known as premature ovarian insufficiency (early menopause). This will cause a lack of a menstrual cycle as their body ovulates irregularly and they have a lack of oestrogen. You can see more about this in our infertility diagnoses

This by no means is an exhaustive list and if you are concerned you have any of these or your periods are irregular, painful or heavy then you should seek medical advice.

Should you track your cycle

The quick answer for this is yes.

My belief is that by tracking your cycle and your symptoms you have a greater understanding of your body and therefore more able to see if something changes. In the context of TTC, it also helps you to pinpoint where you may be most fertile. Cycle tracking can also be used to help in fertility awareness method (FAM) to avoid pregnancy. I first started cycle tracking as a way to predict and be prepared for my period as my cycles varied in length by about 10 days and honestly, we all know what a pain being caught unawares is (halfway up a mountain in Wales anyone?) By understanding when I had ovulated, I could predict my period within a day or two. This is because your luteal phase tends to only change in length by a day either side.

Let me give you an example – my cycles ranged in length from 24 to 34 days. After beginning to track my cycle and trying to detect ovulation I realised that my luteal phase was usually 11 days long (+/- 1 day which is normal). This meant that in general, I tended to ovulate between days 13 and 23. This obviously would significantly change when my most fertile period was but also meant once I had detected ovulation I could just add on 11 days and know when good old aunt flo would make an appearance.

How can I track my cycle?

There are many options for tracking your cycle. Some people with a very regular cycle may purely be able to use the calendar method where you just look at your cycle length and assume you’re most fertile in the middle of your cycle, however, for most women, this will not work. Not only because their cycles have some variability but because their follicular and luteal phases may not be exactly 50% each of their cycle (see my example above). You can find many period tracker apps for your phone in the app stores which can help you would like to use the calendar to monitor your cycles.

Most of us would need to use a combination of methods to track our cycle. You can use basal body temperature (BBT) or core temperature, cervical mucus changes and/or cervical position to track where you are. I personally have used either BBT or core temperature and cervical mucus. There are many apps that can help you with recording this information and understanding it such as Kindara, Flo, Fertility Friend, Glow, Clue any many more. Natural Cycles is the first contraceptive app using FAM to predict fertile days and non-fertile days to tell you when you should avoid sex to avoid pregnancy. To explain this in full is an article in itself, so if you are interested in cycle tracking then please head to my ovulation tracking article.


A women’s menstrual cycle varies from women to women and sometimes from cycle to cycle. However, it all involves the intricate balance oestrogen, progesterone, luteinising hormone and follicular stimulating hormone. If any of these hormones are out of balance this can lead to irregular cycles and problem periods (heavy or painful). If you are any doubt whether your periods or cycle is normal or if your periods or cycle change suddenly you should seek medical advice. Tracking your cycle by a variety of methods may give you a better understanding of what is normal for you and if anything changes. It also may help you avoid or get pregnant if you are looking to do so. Please head to my ovulation tracking article next for information on how you might best track your cycles for any purpose. 

If you have read this far, we hope that this has been useful to you. If you are wanting to learn more I think these are the next articles you should read.

Everything you need to know about Ovulation Tracking – This is a thorough guide to all the ways that you can track ovulation to either get pregnant or try and prevent pregnancy.

A complete guide to cervical mucus – If you are considering trying to get pregnant and you want to get to know your body better, then learn about your cervical mucus.

You can improve your egg quality in 90 days, here is how! – A guide to improving your egg quality for TTC either naturally or through assisted reproduction.

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  1. This blog How does the menstrual cycle work? helped me with my grandmother’s diabetes


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