Most girls start their periods when they're about 12, but they can start as early as 8, so it's important to talk to girls from an early age to make sure they're prepared before the big day.
Many parents feel awkward talking about periods, especially with pre-teen girls, who can seem to get easily embarrassed.
One way round this is to respond to questions or opportunities as they arise. David Kesterton, who organises the FPA's Speakeasy courses – which teach parents how to talk to their children about puberty, sex and relationships – says clear speaking and down-to-earth, age-appropriate language is key.
Best age to talk about periods
"Parents often ask me when the right age to talk to girls about starting periods is, and I recommend that it should be an ongoing process rather than a formal sit-down talk.
"You can use TV ads for tampons, or buying sanitary towels at the supermarket, for example, to start the conversation with girls about periods. Or simply ask your daughter what she already knows and go from there.
"Whenever possible, use clear language, like 'vagina', even though you may feel uncomfortable using these words. Emphasise that periods are completely normal and natural – they're part of growing up and all women have them.
"And don't forget boys. They, too, need to learn about periods. Talk to them in the same way as girls about the practicalities, mood changes that can come with periods, and the biological reason behind periods, and it will keep them informed, as well as help them to understand what girls go through each month."
When a girl starts her periods it's a sign that her body is now able to have a baby. It's important that she also knows about getting pregnant and contraception.
The questions girls ask about periods
Here are some of the questions that you, as a parent, might get asked by girls about periods, with suggestions on how to answer them:
How will I know when my periods are going to start?
Signs that your period is on its way are if you've grown underarm and pubic hair. Typically, you'll start your periods about two years after your breasts start growing and about a year after getting a white vaginal discharge. The average girl will get her first period around 12 years old, but it varies from person to person.
Why haven't my periods started yet?
Your periods will start when your body is ready. That's usually between the ages of 10 and 16.
See a GP if your periods haven't started by age 16 (or 14 if there are no other signs of puberty either).
Possible reasons include being underweight, doing lots of exercise (including dance, gymnastics and athletics) and a hormone imbalance