My daughter just turned 13, and started menstruating 4 months ago. She was well prepared, by mommy (who is a nurse). I am surprised that it was as anti-climactic as it was. I was not even home, she was home with daddy, did what she had to do, and casually told me (when I got home) that she had something to show me. After that, I believe I said (in a question-like manner) "congratulations?". I did not shout it from the rooftops, or post it on facbook or anything absurd like that. She did NOT want my husband to know, she said I could tell grandma (who probably cried) and of course she shared it with grandpa (who really did not want to know).
While I do believe it is a biological and emotional milestone, I didn't feel any need for celebration. No dinner, no card, no present. She didn't see it as a celebration-worthy event. I followed her very matter-of-fact lead. We already had supplies; I just made sure she had enough in her schoolbag now, and we discussed the important things (like hygiene related to it), underwear issues, pants, etc. I reminded her that I am here for any questions (as she knows I always have been). We are very close, so It's not as cold as some may seem. But when you have a medical background, it's easier to get right to the point, and what you are saying (about pregnancy, STDs, etc) is more objective and not as preachy.
While she knew what it was, she really didn't know what it really was. I told her that it means physiologically (not emotionally or financially or in any other way) that your body is preparing for motherhood. The thought of that actually made her cringe (she is into boys, but not nearly that much). To validate that, 2 weeks later, we needed to go to the doctor for a respiratory illness, and they wanted a chest X-Ray, and they asked the big question (last menstrual period). They explained (in a very age-appropriate way) why they need to do a urine sample. I elaborated, and her reply to me was "Mom, I can look you straight in the eye and tell you I am NOT pregnant".
I did reply "welcome to womanhood."
By the second month, she was tired of it. I couldn't help but laugh.