How do to explain to your daughter about periods?

My 9 year old daughter started getting some pains in her abdomen lately, which passes after about 15 mins. Iwas thinking that maybe her body is preparing for the first menstrual cycle. But I have no idea how to explain it to her. I want her to be prepared and at the same time don't want to shock her with any details since she is still so young. Any ideas, please? Thanks

Book

There is book called "whats happenng to me" by Usbourne books. Its been really useful for me having 3 daughters,two are 10 and 12.... I have found it helpful for myself! Explains what happens to your body as it changes and why....

Carol xxx