Tommy's PregnancyHub

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.

What is the menstrual cycle?

The menstrual cycle is what your body goes through to get ready for pregnancy. This cycle is the time from the first day of your period (first day of bleeding) up to the day before the start of your next period. 

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 the sperm.

Ovulation doesn’t happen every month for every woman. There are some conditions that may interfere with ovulation, such as endometriosis and polycystic ovarian syndrome (PCOS). If you are not getting a monthly period and are trying to get pregnant, speak to your doctor.

How long is the menstrual cycle?

Women’s cycles can vary in length and from month to month, but the average is around 28 days. It’s normal to have regular cycles that are longer or shorter than this (from 21 to 41 days).

It doesn’t matter how long your cycle is, most women will ovulate around 10 to 16 days before the start of their next menstrual cycle.

Can I get pregnant at any time during my cycle?

Yes, but it’s unlikely you’ll get pregnant just before or just after your period. You are most fertile when you ovulate, so this is when you’re most likely to get pregnant. Understanding your cycle and knowing more about what is happening month during the month, can help you learn the best days to get pregnant.

Find out more about ovulation and fertility, including top tips for finding your fertility window. You can also use our ovulation calculator to find out more.

What if my cycle is irregular?

It can be more difficult to get pregnant if you have 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 won’t get pregnant and there are things you can do to boost your chances of success.

See your GP if your periods have stopped, you’re missing monthly periods or you have irregular periods and are struggling to get pregnant.

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

This 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 stop altogether. 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.

How soon can I get pregnant after coming off the pill?

You can get pregnant as soon as you come off the pill but if you’re trying to get pregnant, it’s a good idea to wait until you’ve had a natural period. This will give your body time to adjust and give you a chance to make sure you’re ready for pregnancy. Make use of our pregnancy planning tool to see what else you can do.

My menstrual cycle is still irregular. Has the pill affected my fertility?

It’s unlikely the pill has caused any fertility problems, but it can sometimes cover up problems you already have, such as missing periods or PCOS .This is because the pill prevents the ovaries from releasing an egg (ovulation), so although it’s normal to experience period-type bleeding on the pill, you don’t have a ‘real’ period.

Contact your GP if you’re still having irregular periods 3 months 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 therefore when the baby is due) by counting from the first day of your last period.

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

Your pregnancy is calculated from your last menstrual period because every time you have a period this is the body’s way of getting ready for pregnancy. It also gives healthcare professionals a standard to follow because it’s very difficult to know exactly when the sperm fertilised the egg.

If your period is irregular or if you’ve recently been on the pill it will be harder to try and work out your due date.

The most accurate way to work out your due date is to have an ultrasound scan when you’re around 11 to 14 weeks pregnant. This is used to see how for along you are and check your baby’s development.

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

Implantation bleeding sometimes happens in pregnant women around the time their 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) that is 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 Unit immediately to be checked, just in case.

Find out more about bleeding in pregnancy.

Clinical Knowledge Summaries (2016) Contraception – natural family planning!topicSummary

NHS Choices. Periods and fertility in the menstrual cycle (Page last reviewed: 28/07/2016. Next review due: 28/07/2019)

NHS Choices. Irregular periods (Page last reviewed: 09/04/2018. Next review due: 09/04/2021)

NHS Choices. Contraceptive injection (Page last reviewed: 07/02/2018. Next review due: 07/02/2021)

NHS Choices. Contraceptive implant (Page last reviewed: 22/01/2018. Next review due: 22/01/2021)

NHS Choices. When will my periods come back after I stop taking the pill? (Page last reviewed: 17/07/2018. Next review due: 17/07/2021)

NHS Choices. Vaginal bleeding in pregnancy (Page last reviewed: 26/01/2018. Next review due: 26/01/2021)

Review dates
Reviewed: 06 March 2019
Next review: 06 March 2022

This content is currently being reviewed by our team. Updated information will be coming soon.