When your daughter reaches the tween years you will have to sit down and talk to them about puberty and changes they are likely to go through. Talking about periods and menstruation can be quite uncomfortable and awkward for both mum and child, but it is something that can’t be ignored.
Here’s how to do it:
Talk one-on-one
Talk to your daughter on your own without younger siblings around, who may laugh or giggle at what is being said. Show them that you consider them grown up and able to handle the information. Why not make it a real girly experience. However, if your daughter is really uncomfortable talking about this kind of stuff, small conversations over a period of time might be more effective.
Choose the right time
There is no point waiting until she starts her period to have the talk, she will need to know beforehand. Avoid talking about it when she is clearly stressed or when you don't have the time to really explain things. 
Offer practical advice
After the talk many tweens can feel frighten or embarrassed that it could happen at any time. Offer reassurance by giving practical advice like carrying a sanitary pad around with them in their bag and what they should do if they do if it starts while they are outside of the house. Talk about hormones and how their body will change, and tell them how to cope with cramps, so that they can be prepared.
Talk about your own experiences
Talk about your own experiences, so that they know what to expect and know that they aren’t the only ones going through it. Give them relatable experiences so that they will know how to handle the situation when it happens.
Ask if she has any questions
Give her plenty of opportunity to ask questions so that all her concerns can be satisfied. If you don’t know the answer, work together to figure it out, whether it is searching the internet or going to a GP. 
Most girls get their first period when they are around 12 although some can get it as early as nine or as late as 16, so make sure you don't leave it too late.