How much water should I drink before my ultrasound?

Written by diane ursu | 13/05/2017

Ultrasound is used to evaluate many structures within the body. It is most useful in obstetrics and gynaecology because it does not expose the patient to ionising radiation, such as X-rays. It is relatively safe. Many patients are familiar with the "bladder prep"--specific instructions a patient must follow to ensure she has a full bladder for the ultrasound exam.

Types of Exams

Certain exams require a full bladder. Obstetric, pelvic and bladder ultrasounds require the bladder to be adequately filled.

Sonographic Window

Ultrasound cannot be used to visualise structures hidden behind bowel. A full bladder pushes bowel out of the way and allows the sonographer to look at the pelvic organs by using the bladder as a sonographic window.

Fetal Position

For obstetric ultrasounds, a full bladder helps to push the foetus out of the pelvis so it is not hidden by the pelvic bones. It also allows the sonographer to visualise the cervix, the lower portion of the uterus that thins and opens during labour.


Patients are asked to drink 946ml of water. Unless an abdominal ultrasound or other exam that requires fasting is done, coffee, soda and juice may be substituted for water.

When to Drink

First, empty the bladder. Second, drink the suggested amount of fluid within a ten-minute period, one hour before the exam. Do not empty the bladder until instructed to do so.

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