A period is the part of the menstrual cycle when a woman bleeds from her vagina for a few days.
For most women this happens every 28 days or so, but it's common for periods to start sooner or later than this, ranging from day 24 to day 35.
Your period can last between 3 and 8 days, but it will usually last for about 5 days. The bleeding tends to be heaviest in the first 2 days.
When your period is at its heaviest, the blood will be red. On lighter days, it may be pink, brown or black.
You'll lose about 30 to 72 millilitres (5 to 12 teaspoons) of blood during your period, although some women bleed more heavily than this.
Girls have their first period during puberty – 11 is the average age for puberty to start in girls.
A girl's monthly periods usually begin at around the age of 12, although some girls will start them later.
A delay in starting periods isn't usually a cause for concern. Most girls will have regular periods from age 16 to 18.
Sanitary products absorb or collect the blood released during your period. The main types of sanitary products are described below.
Sanitary pads are strips of padding that have a sticky side you attach to your underwear to hold them in place. One side of the pad is made of a special absorbent material that soaks up the blood.
Many women use sanitary pads when they first start their period because they're easy to use. They come in many sizes, so you can change them depending on how heavy or light your period is.
Pantyliners are a smaller and thinner type of sanitary pad that can be used on days when your period is very light.
Tampons are small tubes of cotton wool that you insert into your vagina to absorb the blood before it comes out of your body. There's a string at one end of the tampon, which you pull to remove it.
Tampons come with instructions that explain how to use them. If the tampon is inserted correctly, you shouldn't be able to feel it. If you can feel it or it hurts, it might not be in properly.
It isn't possible for a tampon to get stuck or lost inside you. Your vagina holds it firmly in place and it expands inside you as it soaks up the blood.
Menstrual cups are an alternative to sanitary pads and tampons. The cup is made from medical-grade silicone and you put it inside your vagina.
Menstrual cups collect the blood rather than absorbing it. Unlike sanitary pads and tampons, which are thrown away after they've been used, menstrual cups can be washed and used again.