Endometrial thickening

Hi,
   Thanks so much for answering my question.  I am 49 years old, and have had off and on periods now for several years.  It seems like I had one about every 6 months or so.  This latest one seemed like it had a longer time interval in between, like it had been a year or maybe even longer.  I went to the doctor, and he did a vaginal ultrasound, which showed thickening of the uterine lining-he said it was 9mm.  He said that wasn't crazy thick, but thick enough to do a biopsy.  
   At that point, I did an endometrial biopsy, and so of course now I am very concerned this is cancer.  He was very reassuring, saying that he strongly suspected this would come back benign.  He said he didn't see anything too concering with the US or the biopsy.  I don't know if there are things he can tell look worse than others.  What I wondering is, if this is something benign, wouldn't that have shown up on the vaginal US?  I have read where it could be a polyp, but that should show up on the US, correct?  And, if this IS cancer, is there a good chance it's confined to the Uterus, as I have also read that these types of cancers usually have a good prognosis?  
    I'm concerned because maybe I should have gone into see him when I had irregular periods years ago, but like I said, this one seemed different because it had seemed like a long time in between.  I don't keep very good track, I suppose I should!  
    Is an endometrial biopsy a fairly common procedure?  Or do they only do it when they are highly suspicious of cancer?  He said that my thickening seemed to be only at the top.  Thanks so much for any help!!

[valger59]

Dr.,
   I am adding on to the above post because there's a couple things I forgot to mention.  I have never taken HRT or Tamaxifen, I have three children, I'm 49 years old, I am overweight, and I've been going through menopause for several years now.  Which is why I didn't think it was so unusual to have a period out of nowhere, but then I looked it up on the internet, and went to the doctor.  I'm worried now that I should have gone in years ago, when my periods had long spans in between them.  I have always done my regular paps, and just 3 months ago, had a normal pap and mammogram.  Even though some blood tests years ago suggested I was in menopause, I thought it was still normal to have irregular bleeding, since I am fairly young, and I feel like I've had hormonal problems for a while, with heart palpitations, etc.  I just wanted to add that info!  It's hard waiting for the results of the biopsy!  Thanks, Valerie