Hi Katejo,
As mentioned above, the full bladder is for transabdominal pelvic ultrasound scan (TAS), but it's not just to push the womb forward… The ultrasound waves travel at different speeds through different tissues and gas will prevent them from reaching anything behind it. The full bladder will push the intestines (usually full of gas) out of the way AND will provide a clear fluid ‘window' for the ultrasound waves to reach the womb, because sound waves travel very well through fluid (same reason they use the gel on your skin).
I hope both of you get the transvaginal scan (TVS) as well, so you will probably have TAS with a full bladder and right after that a TVS with an empty bladder, as Mogster71 has explained. Many studies show that TAS is less accurate and should not be used for endometrial assessment, not to mention that postmenopausal shrinking ovaries and polyps cannot be assessed through TAS.
BeaR.