I only just started my spell
Answers: Don't worry. Your first year will be unpredictable any way. .
Well, from the bearing you asked your question, you sound a bit freaked out so first off: calm down!
Since you purely started you're gonna be a little spontaneous with the timing but since your mom know she'll understand. I'm not too sure whether you're afraid that it's not going to start on time or that your mom think it's coming at a different time...? Either way don't worry I'm sure she'll take in and don't stress too much about your period earlier it comes..
When I first started my period, I started in January, and didn't gain it again until May. But that was me, and when you first start your period, its highly unpredictable. But to be safe, carry around pad or tampons just in grip :] I'm sorry I don't understand your question.