Menstruation :: No Period At The Age Of 17?

My daughter just turned 17 a few weeks ago and still has not gotten her period. I got mine when I was around 15, so I find this a bit odd. She has had pubix hair since she was around 11ish. She got armpit hair around 12 and breasts at around 13, but still no period. She has had a yeast infection before and it was treated. She has also had an ultrasound because when the gynecologist tried to check out her insides she couldn't feel her cervix because my daughter was squirming too much. After the ultrasound they confirmed that she had one and everything seemed fine. My daughter has never had sex, so I know she is not pregnant. My daughter is also on the heavier side. She is slightly overweight. I say this because I know when women are too skinny sometimes their periods don't show up. Ever since she was around 13 ever doctor has told us she will get her period in 6 months or so. 4 years later and that still hasn't happened. She is starting to get worried.