Why does my girlfriend have missing periods?

My girlfriend has been missing her period for many months.
Like about 6 or 7 now.
She didn't start having it till she was like 15 and a half.
It has been about 2 years since then.
She went a long time without having one at all.
Then she had one. After...

that it would stop for a long time.
But then she had it for like 3 months straight.
Then she hasn't had it since.
So and info can/will help.
Thank you

Irregular periods are very common, especially in the first three years that girls have their period. However, she should mention it to her ob-gyn just to be on the safe side.

She did once go to one.
But the lady said it is normal for the first year or so.
But is there anything she can do to help it?

She could go on the birth control pill. Many girls go on the birth control pill to regulate their period. The hormones in the pill make the girl's period come every month, and they make her period a lot more predictable. The pill also can make periods lighter, shorter, and reduce cramps.

Thats good to know and thank you.
But since we are only 17.
Her mom wouldn't let her get it.
Don't you have to be 18 to get it?

No, she does not have to be 18. Girls can get the birth control pill at any age. She would have to make an appointment with Family Planning or Planned Parenthood. They give girls birth control even if they are underage and even if their parents do not approve.

Family Planning and Planned Parenthood will go out of their way to make sure parents don't find out that the girl even goes there unless the girl wants them to know.

If you visit this site: http://ww.plannedparenthood.org/ You can put in your zip code to find the Planned Parenthood that is nearest you.

OK WAIT an unregular peiod can ALSO be caused by stress make sure hes not under alot of pressure..i recomend seeig an obgyn also .. my mom had unregular periods and she got tumors later on in life and had 13 stillborn children.. plz see a doctor