The length of a menstrual cycle varies from person to person. A period usually comes every 28 days, but it's normal if it's longer or shorter than that. Everyone is different.
The menstrual cycle
Your body's hormones control the menstrual cycle. When the level of the oestrogen hormone in the body increases, the ovaries will release an egg (ovulation).
In the second half of the cycle, the levels of progesterone hormone will increase and this gets the womb ready for pregnancy.
If pregnancy doesn't occur, oestrogen and progesterone levels will drop. This hormonal change causes the lining to be shed from the womb which causes a period.
Find out more about periods
Ovulation occurs when an egg is released from the ovaries.
One egg will be released each month during the menstrual cycle. An egg will live for 24 hours after ovulation.
If a male’s sperm meets and fertilises the egg, this will cause a pregnancy. After having sex, sperm can live for up to 5 days inside the fallopian tubes. If you don't want to get pregnant, use contraception.
It’s not possible to get pregnant if ovulation doesn't occur.
When you’re most fertile
You’re most fertile around the middle of the menstrual cycle at the time of ovulation. This is known as the 'fertile window'.
This will depend on the length of your cycle. It usually happens around 14 days before your period starts.
There are different ways you can track your cycle if you want to work out when you are ovulating to plan a pregnancy. This shouldn’t be used as a method of contraception. This includes:
- keeping a diary, or marking the dates of your period on your calendar
- using an ovulation calculator
- using a period tracking app – though this is not effective as a type of contraception
- monitoring your temperature and noticing changes in your vaginal mucus
- using an ovulation prediction kit