With our first child, I had an ultrasound at about 26 weeks and was not required to drink any water. This time, at 20 weeks and with a new doctor and lab, I was told to drink lots of water and not to eat certain foods after 9pm the day before.
This can vary a bit from site to site but, in general, early ultrasounds -- prior to 20 to 24 weeks or so -- do require a full bladder when the transabdominal approach (transducer is placed on your abdomen) is used.
The reason for this is that the fluid in the bladder creates a "window" for the beam to pass through. (This provides a good medium for sound conduction.) It also serves as a "landmark" for the technician to get their bearings, so to speak. In addition, a full bladder can change the position of the uterus, taking the flexion out of it and pushing it up so it is easier to scan. A full bladder also moves loops of bowel up and out of the way to make the pelvic organs easier to view.
In later pregnancy, or with a transvaginal approach, (the probe is introduced into the vagina), a full bladder is not necessary. The enlarging uterus is quite nicely positioned above the pubic bone and the fluid within the sac provides the medium for passage of the beam.
