A full bladder is needed during an ultrasound because it helps to provide the best image, as stated by the National Institutes of Health. When the bladder is full, the uterus is moved higher in the belly, which allows for an unobstructed view of the baby.
During early pregnancy, a full bladder also helps to move the loops of the bowel out of the way. The bowel contains gas which can impact how well the ultrasound waves travel. In order to ensure the bladder is full, it is recommended that the patient empty the bladder approximately two hours before the ultrasound. Once the bladder is empty, the patient should drink three or four glasses of water in the hour leading up to the test. This procedure is only necessary for an abdominal ultrasound. Prior to a vaginal ultrasound, the patient is instructed to empty the bladder.
An ultrasound is performed during pregnancy to determine whether or not there are problems with the fetus, how far the pregnancy has progressed and to measure the amniotic fluid within the sac. The ultrasound is usually done in conjunction with other tests, such as blood tests, to detect the presence of Down syndrome. Ultrasounds are considered safe and have no risks associated with them.