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Ultrasound is used to evaluate many structures within the body. It is most useful in obstetrics and gynecology because it does not expose the patient to ionizing 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

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

Sonographic Window

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Ultrasound cannot be used to visualize 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

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


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Patients are asked to drink 32 oz. 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

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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.