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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 three and eight days, but it will usually last for about five days. The bleeding tends to be heaviest in the first two 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.
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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.
Read more about girls and puberty.
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.
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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.
Changes in your body's hormone levels before your monthly period can cause physical and emotional changes.
This is often known as premenstrual syndrome (PMS) or premenstrual tension (PMT).
There are many possible symptoms of PMS, but typical symptoms include:
These symptoms usually improve when your period starts and disappear a few days afterwards. PMS doesn't affect all women who have periods.
Working out when you can get pregnant – your fertile time – can be difficult. It's around the time you ovulate, which is about 12 to 14 days before the start of your next period.
However, sperm can survive inside a woman's body for days before ovulation occurs. This means your fertile time extends back earlier in your cycle.
You can calculate when your period will start and your peak ovulation times using an online period calendar.
Your periods can change – for example, they may last longer or get lighter. This doesn't necessarily mean there's a problem, but it does need to be investigated.
You can go to see your GP, or you can visit your nearest women's clinic or contraceptive clinic.
Bleeding between periods, bleeding after having sex, or bleeding after the menopause needs to be checked by a doctor. It might be caused by an infection, abnormalities in the neck of the womb (the cervix) or, in rare cases, it could be cancer.
You could be pregnant if you miss a period and you've had sex. See your GP if you've taken a pregnancy test and the result is negative (you're not pregnant) and you've missed three consecutive periods. Your GP will investigate the cause and recommend any necessary treatment.
Read more about stopped or missed periods.
Your periods will continue until you reach the menopause, which usually occurs when you are in your late 40s to mid-50s. In the UK the average age of menopause is 51.
Your periods may start to become less frequent over a few months or years before stopping altogether. In some cases they can stop suddenly.