Once a girl starts having periods this is a sign that she is able to get pregnant, regardless of how old she is.
Many people don't realise that it's possible to get pregnant if sperm comes into contact with your vagina, and that you don’t have to be having unprotected sex for this to happen.
Conception is a crucial part of the process of having a baby. Whilst by itself this is a straightforward process, there are lots of different things which can affect the likelihood of a woman becoming pregnant, such as smoking, diet, weight and other health conditions.
Here's how conception works:
As girls get older and mature into women, their bodies start releasing the eggs which will enable them to become mothers. These eggs are released from the ovaries and travel to the uterus (womb) via the fallopian tubes, where it is possible that they may meet sperm if the woman has unprotected vaginal sex.
Women's ovaries release an egg every month - this causes the body to prepare for a possible pregnancy. The main way it does this is by releasing lots of hormones and by thickening the lining of the womb for the egg to embed in if it gets fertilised.
If the egg does not get fertilised the body gets rid of the egg and the womb lining. This process is called menstruation, or having a period.
As boys get older and mature into men their bodies start producing sperm, which will enable them to become fathers.
Sperm are produced in the testicles (balls) and are released out of the penis through ejaculation.
To help carry the sperm to reach an egg, they are released in a whiteish liquid called semen.
Millions of sperm are released when a man ejaculates, but it only takes one sperm to fertilise an egg.
During unprotected vaginal sex sperm may get the chance to reach an egg and cause fertilisation, as it can travel through the vagina to the womb. This is most likely to happen when a man ejaculates inside a woman, because there will be millions of sperm swimming to meet the egg.
However, it could also happen if a man has sex and does not ejaculate. This is because as soon as a man gets an erection (hard-on), he produces a small amount of fluid called pre-cum and it is possible that this might contain sperm.
Once inside the vagina, the sperm in the pre-cum could also go and meet an egg. Remember, it only takes one to fertilise the egg!
What happens in the womb
Conception occurs when sperm from a man meets an egg from a woman and these two cells combine to create a fertilised egg, which then has the possibility of becoming a foetus.
A foetus is the first stage in the growth of a baby in a woman's womb. If you have a foetus then you're pregnant. If the foetus grows into a baby and there is no miscarriage or abortion, then nine months later a baby will be born.
This process usually happens through penetrative vaginal sex, but may also happen with the help of doctors inside or outside of the body.
Conception occurring does not mean that the egg will definitely become a baby, as it has to develop a lot more for this to happen. Sometimes eggs are fertilised and do not develop enough to become a foetus.
When is it safe to have unprotected sex?
There are times of the month when it is easier for a woman to get pregnant.
Once an egg has been released it only lives for around one day; but it is difficult to know exactly when an egg has been released.
No time of the month can ever be seen as a safe time to have sex because of this uncertainty.
Even if your periods are regular, you can randomly release an egg at a time which is not usual for you and unexpectedly become pregnant.
Since sperm can survive inside your body for up to seven days, it is possible that it could still be around to fertilise any eggs released after you have had sex. It is difficult to know when these processes are happening - so it is always best to use some kind of contraception if you do not want to become pregnant.