Q.My 13-year-old daughter started her period recently and it is throwing her for a loop.

She has had sex education in school, talks easily with her father and me and is an extremely confident child, but this has taken her aback. She even asked me if she could still play basketball.

Is there a book that lays out some of the biological facts, written for someone her age, so we could discuss it together? She's a good reader but perhaps a little immature emotionally.

I need a book that answers the emotional and moral questions but especially the physiological stuff, since my daughter may not have absorbed the information she received in the past. I have often heard that children don't learn something until they're ready to learn it and this may be an example.

A.Any change is unsettling, but the transition from childhood to adolescence may be the most traumatic change your daughter has known.

Quite naturally she blames her new uncertainties on her period even though her hormones are what's making her feel so discombobulated.

Suddenly her body is telling her to leave childhood behind whether she's ready or not. No matter how much she fights it, she now feels silly when she plays with the dolls she once adored or when she jumps rope or plays hopscotch with the little girl next door.

Moreover, some of your daughter's friends are acting like they're 13; some are acting like they're 14, 15 or even 20 and others are still stuck in the preteens, which must make her wonder if her buddies will ever be friends again. This is sure to make her feel anxious and insecure and the next thing you know, she will start worrying about school and how it will be next year, what people and what pitfalls she should avoid and how big her breasts will be. Who wouldn't be anxious?

Even if your daughter expected menstruation to begin when it did and knew exactly why it would happen, she may not be ready for it. And that's all right. She just needs enough time to adjust and enough information to make sense out of puberty, but don't expect her to remember what you told her about sex last year, for learning is layered. A child -- any child -- understands and remembers best if a subject is taught over and over again, with some new information added each time, than if the entire lesson is taught to him all at once.

Your plan to read a couple of good books with your daughter on sex is a good one, particularly if you read "The What's Happening to My Body? Book for Girls" by Lynda Madaras with Area Madaras (Newmarket; $12.95) and "The Period Book" by Karen Gravelle and Jennifer Gravelle (Walker and Co.; $8.95), but you'll need more help than that.

You may be able to teach sex ed easily to other children, only to find yourself downright embarrassed when you have to teach your own, especially when she gets older. To avoid a case of the shies, either now or later, do your talking in the dark, so you don't have to make eye contact, and talk into a tape recorder so you can practice your answers. These techniques will be especially helpful when your daughter asks you questions like "How far can I go? Should I give in? Why not? Then when?"

To answer these questions with the thoughtfulness they deserve, study some good books on sex that are written for parents. They will help you consider, and reconsider, your answers. The best may be "Sex and Sensibility" by Deborah M. Roffman (Perseus; $16.95); "The Big Talk" by Laurie Langford (Wiley; $14.95) and "Beyond the Big Talk" by Debra W. Haffner (Newmarket; $14.95). These books will help you define your values and refine your advice so your daughter will want to ask you what she should do, rather than her boyfriend, and that's just what you want. His advice almost surely will be different from yours.

Questions? Send them to advice@margueritekelly.com or to Box 15310, Washington, D.C. 20003.