We must get at least a dozen comments and emails every week from girls asking the question, “When will I get my first period?” It’s obvious that every girl has put a lot of thought into how puberty is progressing and what’s up next because we get all sorts of details. We hear about when breasts budded, how long pubic hair has been around, whether there’s discharge or not, when mom started her period, and how many friends have already started. Some girls seem worried, while others are clearly excited and impatient!

So how do you know? There are definitely some things that should happen BEFORE you start your period, but the timing is a bit of a mystery. Wouldn’t it be cool if you got a “reminder” that popped up on your cell phone?...