Understanding your menstrual cycle

Getting to know your menstrual cycle can help you understand when you’re most likely to get pregnant during every month.

Getting to know about your menstrual cycle can help you pinpoint when you’re most likely to get pregnant during each month. 

What is the menstrual cycle?

Each menstrual cycle starts from the first day of your period (first day of bleeding) up to the day before the start of your next period. During these cycles, your body is doing things to get ready for a possible pregnancy.

Cycles can vary in length from person to person. The average cycle lasts around 28 days but it’s normal to have regular cycles that are longer or shorter than this (from 23 to 35 days).

Ovulation usually happens during the second half of the cycle. This is when an egg is released from one of your ovaries. To get pregnant, the egg needs to be fertilised by sperm. 

It doesn’t matter how long your cycle is, most people will ovulate around 10 to 16 days before the first day of their next period, when the menstrual cycle starts again.

Ovulation doesn’t happen every month for some women and birthing people. There are some health conditions that may stop it, such as endometriosis and polycystic ovarian syndrome (PCOS). If you are not getting a period every month or so and are trying to get pregnant, speak to your doctor.

Can I get pregnant at any time during my cycle?

Yes, but you’re least likely to get pregnant just before or just after your period. You are most fertile when you ovulate, meaning this is when you’re most likely to get pregnant. Getting to know your cycle can help you learn the best days to get pregnant.

What is my menstrual cycle timeline?

The following is based on a 28-day cycle. You'll have to adjust this depending on how much shorter or longer your cycle usually is.

  • Days 1 to 7: You have your period, which can last from 3 to 7 days.
  • Days 7 to 10: The lining of your womb gets thicker, ready for a possible pregnancy.
  • Days 10 to 16: Your fertile window.
  • Days 13 to 15: Your ovaries release an egg (ovulation). No matter how long your cycle, you will probably ovulate around 10 to 16 days before your next period starts.
  • Days 16 to 25: The womb lining carries on thickening.
  • Days 26 to 28: If you’re not pregnant the womb lining starts to come away again, and comes out as your next period.

Find out more about ovulation and fertility, including top tips for finding your fertility window.  

What if my cycle is not the same length each month?

It can be harder to get pregnant if the length of your cycle differs by a lot each month (irregular periods) because you may not ovulate regularly.
There are many possible causes of irregular periods, which may affect your fertility. For example:

Having irregular periods does not mean you will not get pregnant. There are things you can do to boost your chances of success.  But see your GP if you have irregular periods and are struggling to get pregnant, or if your periods are missing or have stopped.

I’ve been using contraception. Will this affect my cycle?

It depends on what type of contraception you’ve been using. If you’ve been taking the pill, your period may be irregular when you first come off it, so try to give yourself up to 3 months for your natural menstrual cycle to get back to its normal routine. 

The first period after stopping the pill is known as a ‘withdrawal bleed’. The next one after this is your first natural period.

The contraceptive injection can also affect your cycle. Your periods may change and become irregular, heavier, shorter, lighter or even stop. This can carry on for some months after you stop the injections.

If you use the contraceptive implant your fertility will return to normal as soon as it is taken out.

Contact your GP if you’re still having irregular periods 3 months after stopping contraception. 

Find out more about getting pregnant after stopping contraception.

Can I use my menstrual cycle to predict my due date?

Yes. You can work out how far along your pregnancy is and when the baby is due by counting from the first day of your last period

This can be confusing because you probably didn’t actually get pregnant until around 2 weeks later, around the time you ovulated. But, even if you know the actual date you got pregnant, this is still counted as day 14 of your pregnancy (if your cycle is 28 days), not day 1. 

Your pregnancy is calculated from your last period because your period this is your body’s way of getting ready for pregnancy. It also gives healthcare professionals the same standard to follow when it’s difficult to know exactly when your egg was fertilised. 

If you’re not sure when your last period was because they’re irregular or you’ve recently been on the pill, it will be harder to work out your due date. 

However, your ultrasound scan when you’re around 11 to 14 weeks pregnant will be used to more accurately work out your due date. It can see see how far along you are and check your baby’s development.

I’m pregnant but have some bleeding. Is this my period?

Implantation bleeding sometimes happens when you’re pregnant at around the time your period would have been due. Implantation is when the developing embryo plants itself in the wall of your womb.

An implantation bleed is very light bleeding (spotting). It’s usually pinkish and sometimes brown. Not everyone will have an implantation bleed. 

It’s common to have light bleeding or spotting without pain before 12 weeks. This isn’t often serious, but you should contact your doctor, midwife or Early Pregnancy Assessment Unit (EPAU) right away to get checked over, just in case.

Find out more about bleeding in pregnancy.

NHS (2024) Contraceptive injection. Available at: https://www.nhs.uk/contraception/methods-of-contraception/contraceptive-injection/what-is-it/ (Accessed 20 March 2024) (Page last reviewed: 09/02/2024. Next review due: 08/02/2027)

NHS (2024) Contraceptive implant. Available at https://www.nhs.uk/contraception/methods-of-contraception/contraceptive-implant/what-is-it/ (Accessed 20 March 2024) (Page last reviewed: 28/02/2024. Next review due: 28/02/2027)

NHS (2021). Due date calculator. Available at: https://www.nhs.uk/pregnancy/finding-out/due-date-calculator/ (Accessed 20 March 2024) (Page last reviewed: 29/04/2021. Next review due: 29/04/2024)

NHS (2021). Vaginal bleeding. Available at: https://www.nhs.uk/pregnancy/related-conditions/common-symptoms/vaginal-bleeding/ (Accessed 20 March 2024) (Page last reviewed: 10/03/2021. Next review due: 10/03/2024)

NHS (2022). Irregular periods. Available at: https://www.nhs.uk/conditions/irregular-periods/ (Accessed 20 March 2024) (Page last reviewed: 26/07/2022. Next review due: 26/07/2025)

NHS (2022). When will my periods come back after I stop taking the pill? Available at: https://www.nhs.uk/conditions/contraception/when-periods-after-stopping-pill/?tabname=methods-of-contraception/ (Accessed 20 March 2024) (Page last reviewed: 01/03/2022. Next review due: 01/03/2025)

NHS (2023) Infertility: causes. Available at: https://www.nhs.uk/conditions/infertility/causes/ (Accessed 20 March 2024) (Page last reviewed: 09/08/2023. Next review due: 09/08/2026)

NHS (2023) Periods and fertility in the menstrual cycle. Available at: https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/ (Accessed 20 March 2024) (Page last reviewed: 05/01/2023. Next review due: 05/01/2026)

NHS (2023) Periods. Available at: https://www.nhs.uk/conditions/periods/ (Accessed 20 March 2024) (Page last reviewed: 05/01/2023. Next review due: 05/01/2026)

NICE Clinical Knowledge Summaries (2021) Contraception – natural family planning. Available at: https://cks.nice.org.uk/contraception-natural-family-planning#!topicSummary (Accessed 20 March 2024) (Last reviewed 06/2021)

Review dates
Reviewed: 22 March 2024
Next review: 22 March 2027