Once she has started to menstruate, a girl can fall pregnant if she has unprotected sex (without a condom or other contraception). You’ll get your period every 21-35 days or so. It is often a little different each cycle.

The cycle can be divided into two parts: Pre-ovulation and post-ovulation. Ovulation is when the ovary releases the ripe egg.

Pre-ovulation starts with the first day of menstruation and lasts until ovulation. It can be from 9 to 23 days long. The first day of menstruation is the first day of your menstrual cycle.
Menstruation lasts 3-7 days. During menstruation, a hormone (chemical) causes eggs in your ovaries to start to ripen. The sac that holds the ripening egg releases another hormone (oestrogen) that causes the lining of the uterus to start to build up. Ovulation can happen anywhere from 3-17 days after menstruation ends.

When the egg is ready, your ovaries will release it – this is called ovulation. It will move down your fallopian tube towards your uterus.

If you have unprotected sex, the boy or man’s sperm enter your vagina and make the journey through the cervix and uterus into your fallopian tubes looking for an egg. If they meet, the egg becomes fertilised in the fallopian tube. It then travels down the fallopian tube and attaches itself to the lining in your uterus. If the egg is not fertilized, it will die and be absorbed into the body. If the egg dies, the level of hormones go down and that tells the uterus to get rid of the lining it has built up. This the first day of your next period. And the menstrual cycle starts all over again.

Each girl and woman has her own cycle. When you start menstruating, it is a good idea to write down the first day of bleeding each cycle in a small notebook and keep track of it. You can track your period with a free app like Pink Pad, Monthly Pal or My Cycles. Search your phone's app store for any of these.

Once you menstruate it means you can fall pregnant any time you have sex without using a condom or other methods to prevent pregnancy.