Hi Sarah,
I seem to have had loads of those in my time, and depending on where you are at in your cycle, they are checking for different things. Basically, seeing as where you are at in your cycle, they are going to check for exactly what Deb said - follicle and lining. If it's your first they might just be checking that everything looks just how it should. They always tell me that i have a retroverted uterus when i have one done, and once they noticed a large cyst, as well as a follie and they checked back on day 3 to see if the cyst had gone (and it had).

Just thought i'd add that at my first clinic they always put a condom on the end of the ultrasound - it's hygenic and just the right size. At my regional clinic now, they stick a surgical glove on it and put the ultrasound right up into the finger of the glove. THEN DH was horrified at Freemasons the other day when the Dr put gladwrap on the end of it!!!! We figured money must have been tight that week!

Good luck Sarah, hope they find it all looking just perfect.