And Samm? Moms are smart. They keep an eye on stuff like missing pads and tampons, what's in the trash, bloody Kleenex, extra-hungry kids, mood swings and complaints about cramps and headaches. I'll bet you anything your mom probably knows perfectly well that you haven't started yet (although she might have forgotten because it's not a reason to worry at your age), so don't be afraid to talk to her about it.
You're only 15, so really you're right in the ballpark for it to start. I don't think there's anything wrong with you at all! And believe me, periods aren't any fun, so don't be in a huge rush to get one.
Like Ashelen said, if you're sexually active, or planning to be, you should see a gyno first so you can get checked out and find out about safe sex options that are right for you. There is NO rush. Most girls really aren't ready for all the emotional baggage and responsibilities that can come with sex (like damaged reputations, unplanned babies, diseases, angry families and getting teased at school) at 15.
I know girls who got theirs in third grade (!) and women who got them in their late teens. I don't mean to scare you, periods are totally natural for women. But mine came at 13, and I wasn't one of the lucky ones who enjoy a breezy, symptom-free cycle. It's painful and turns me into a very angry person who can't keep her hands off the chocolate. It gets bad enough that I've had to miss work. I would have LOVED to have had a couple extra years without that experience.
Your time will come soon enough. :-)
First, take a few breaths! Yes it can be entirely normal to not have had your period yet. Are you sexually active yet?
I don't think your mom will worry one bit; women used to not get their periods until they were 15-17 on average, it's only in the last 30 years or so that women have gotten them younger and younger. I got mine at 11, but one of my best friends got hers at 16. And when you start your period has absolutely no effect on if you can have kids in the future or not!
Usually girls wait until they get their first period to see the gynecologist, but if you're worried, ask your mom to make you an appointment and you can ask them yourself to make sure your hormone levels are all A-OK, but I really think you're just fine.
If you are sexually active, it changes things a bit, and not having your period could be a bad sign (as in a sign of pregnancy or an STD) but if you AREN'T active yet, I wouldn't worry and just enjoy this time! Once your period starts, it's every month for the rest of your life and it's really not any fun, lol.