Mammogram and Ultrasound Result

I am 35 years old and just recently had my baseline mammogram (3-27-2008).  I was called back to have additional views of the left breast taken due to "density".  I had additional views on 4-9-08 due to three areas in question (I believe they referred to them as fibroglandular nodules).  The radiologist recommended an ultrasound following the results of the mammogram.  I had the ultrasound and they did not find anything (cysts, ect).  I followed up this week with my OB and she said they catergorized it as a Briads 3.  Why did they see something on the mammogram and not on the ultrasound?  Is this typical?  She also felt my left breast and said she said it just felt like fibrocystic changes.  Should I follow-up with a breast specialist or wait for the 6 month repeat mammogram?  Are they just being cautious because it was my baseline mammogram and don't have anything to compare it to?  Thanks for the information.

[Cleveland Clinic]

Dear Autumn2673, It is not unusual that something from a mammogram doesn’t show itself on an ultrasound.  Different tests give different types of information.  An ultrasound gives information that is better at distinguishing between something that is fluid-filled (cyst) or solid.  A BIRAD 3 means that the abnormality is likely benign and that follow up would be recommended in 6 months.  It may be that they are being extra cautious because there is nothing to compare it to.  Whether you follow up with a breast specialist is your decision, based on your comfort level with the recommendations by your OB and radiologist.