This article is from the Pregnancy Screening FAQ, by Lynn Gazis-Sax (firstname.lastname@example.org) with numerous contributions by others.
Women getting an ultrasound are encouraged to drink several glasses of
water an hour before the exam and not go to the bathroom until after the
exam. The full bladder helps the doctor locate the pelvic organs and get
a clearer and more accurate picture. However, this advice only applies
for some ultrasounds, depending on the kind of probe used and the point
in pregnancy when the ultrasound is done. This is why different women
report getting different advice from their doctors about whether a full
bladder is required.