The length of a woman's menstrual cycle (the length of time between periods) can vary from between 24 to 35 days but, on average, it lasts 28 days.
Ovulation (when an egg is released from your ovaries into your fallopian tube) occurs approximately half way through your menstrual cycle - roughly 14 days before the start of your next period.
The days around ovulation (between 10 to 16 days before your next period) are when you are at your most fertile. So, if you are trying to conceive (get pregnant), this is the best time of the month to have unprotected sex with your partner.
During the middle of your menstrual cycle, the female hormone called oestrogen, produces another hormone called luteinising hormone (LH). The increased level of LH causes a follicle (a small, fluid-filled sac in your ovary) to release an egg. Within 24 hours of the egg being released, it is ready to be fertilised by a man's sperm.
After ejaculation, sperm can live for at least 48 hours, and up to seven days inside your vagina or womb after sex. This means that it is possible for you to become pregnant if you have unprotected sex two days before, or a couple of days after, ovulation - that is, between day 10 and 16 of your menstrual cycle. The least likely time that you will conceive is between days 1 and 7 of your menstrual cycle.