Before your daughter has her first menstrual period, tell her what to expect. While most girls start menstruating at the age of 12 or 13, many others start as early as nine or as late as 16. Buy your teen a supply of pads or tampons and show her how to use them and how to dispose of them after use (fold, wrap up in toilet paper or in the lining the next pad comes with, and put in the garbage). Remind her not to try to flush the used pads down the toilet unless she's prepared to pay the $250 plumber's fee. Try to give a positive spin to the whole process by sharing your amazement at the miraculous way that the human body works and tell her that it's a sign that she's becoming a woman. Remind her that the first 10 years or so of her life, you took care of her, and now the years from 10 to 20 are years during which she'll learn to take care of herself before she eventually gets married and takes care of other people.
Tell your daughter about the other changes she'll be undergoing, such as breast development, increased activity of sweat and oil glands, with their results of body odor and acne; take your daughter shopping (teenage girls love going shopping) for a training bra, deodorant, and acne cream, if she needs it. If one breast is larger than the other, reassure your daughter that the other one will catch up. Prepare your daughter for the growth of pubic and underarm hair, and if she's bothered by hair on her legs, offer her the choices of shaving, depilatories or waxing.
Again, give over the information in a positive way, stressing that adolescence is a normal stage of life and you're proud of her.