Muslim mothers should teach their daughters about menstruation—the biological explanation of it as well as the rules of purification—before the girls attain maturity. There is nothing to be shy about discussing menstruation with your daughter.
Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states: There is nothing to be shy about discussing menstruation with your daughter as a Muslim mother. The Prophet (peace and blessings be upon him) said, “There is nothing to be shy in seeking (or discussing matters of) knowledge.” We know that the Muslim women during the Prophet’s time were quite frank about asking him questions related to such matters without any qualms or inhibitions whatsoever.

If, however, you are too shy to talk to your daughter about issues like this, or you find yourself inhibited or unable to answer her more detailed queries on such matters, then your best bet is to seek help of other Muslim women preferably those who are teachers or female Muslim doctors to explain things to her. If she is attending Islamic classes, let a knowledgeable and qualified female teacher have a session about such issues with girls of her own age group. The sooner you do this the better, for it is important for her to know about this issue before she experiences it. Alternatively, you can get some good books which may deal with such topics in a clear way that children of her age group can readily grasp. Here is a list of books I suggest that you get as soon as possible to make them available for your daughter:
For Heaven’s Sake by Ruqaiyyah Waris Maqsood (Tahah Publications, Ltd.)
Living With Teenagers by Ruqaiyyah Waris Maqsood (Tahah Publications, Ltd.)
What Islam Is All About by Yahya Emerick.