What you need to do before an ultrasound depends entirely on which type of scan you’re having. Some require fasting, some require a full bladder, and others need no preparation at all. The most important step is confirming your specific instructions with the facility scheduling your scan, but here’s what to expect for the most common types.
Abdominal Ultrasound: Fasting for 6 Hours
If your ultrasound is examining organs in your abdomen, like the liver, gallbladder, pancreas, or bile ducts, you’ll need to stop eating for at least six hours before your appointment. This keeps the gallbladder filled with fluid so it shows up clearly on the image, and it reduces the amount of gas in your digestive tract that can block the view.
The timing works differently depending on whether your scan is in the morning or afternoon. Johns Hopkins Medicine recommends eating a fat-free dinner the evening before a morning appointment, then nothing to eat or drink until after the exam. For an afternoon appointment, you can have a clear liquid breakfast (no milk) before 9 a.m., then nothing after that.
Small sips of clear water are generally allowed up until about two hours before the scan, but check with your facility. “Clear fluids” means water, black coffee, or clear juice, not smoothies or milk-based drinks.
Pelvic Ultrasound: Fill Your Bladder
A transabdominal pelvic ultrasound, the kind where the probe moves across your lower belly, requires a full bladder. The fluid acts as a window that helps sound waves travel to the uterus, ovaries, or bladder, producing a much clearer image than an empty bladder would allow.
The standard instruction is to drink 32 ounces of water (about four glasses) and finish it one hour before your appointment time. Then, critically, don’t urinate. This is the part most people underestimate. A full bladder for an hour can be uncomfortable, so time it carefully. Arriving too early with a full bladder just extends the discomfort, while drinking too late means the bladder won’t be full enough and you may need to wait longer in the office.
If your scan includes a transvaginal portion (where a smaller probe is inserted vaginally), the technician will typically do the transabdominal images first with your full bladder, then let you empty it before the transvaginal part. Transvaginal scans work better with an empty or nearly empty bladder.
Pregnancy Ultrasound: It Depends on the Trimester
Early pregnancy ultrasounds, particularly in the first trimester, often require a full bladder using the same 32-ounce protocol. The uterus is still small and sits deep in the pelvis at that stage, so the fluid helps with visibility.
By the second and third trimesters, the uterus has grown large enough that a full bladder is usually unnecessary. You can eat and drink normally before these appointments. The scan itself typically takes about 30 minutes. Most practices schedule a follow-up right after so you can discuss the results the same day, though in some cases a radiologist or obstetrician reviews the images and shares findings at your next regular appointment.
Vascular (Doppler) Ultrasound
Doppler ultrasounds check blood flow through your arteries and veins. Preparation varies by the specific type. An abdominal Doppler, like one checking the aorta or renal arteries, typically requires fasting just like a standard abdominal scan. Carotid or leg Dopplers usually don’t require fasting.
One requirement that catches people off guard: if you smoke or use nicotine products, you’ll need to stop at least two hours before the test. Nicotine constricts blood vessels, which can directly affect the measurements the scan is trying to capture and skew your results.
What to Wear
Wear comfortable, loose-fitting clothing, ideally a two-piece outfit. You’ll need to expose the area being scanned, so a separate top and bottom makes this easier than a dress or jumpsuit. For a breast ultrasound, you’ll be asked to undress from the waist up and wear a gown.
Leave jewelry at home if it’s near the scan area. Necklaces for a thyroid or carotid scan, belly button piercings for an abdominal scan, and earrings for a head or neck scan may all need to come off. Keeping them at home avoids the risk of losing them at the facility.
Medications and Fasting
If you take daily medications for blood pressure, heart conditions, pain, or seizures, you can generally still take them on the morning of a fasting ultrasound with a small sip of water. The key exception is diabetes medication. Oral diabetes pills are typically held during the fasting period, and if you use insulin, your provider may adjust your dose. Check your blood sugar the morning of the scan so you can report it when you arrive.
Preparing Children for an Ultrasound
Fasting guidelines for children are shorter than for adults and depend on age. For infants under six months, the fasting window before a scan may be as short as three to four hours. Older children generally follow closer to the adult six-hour guideline for solid food, though clear fluids are often allowed up to one or two hours beforehand. Your child’s care team will give you age-specific instructions.
Beyond the physical prep, helping younger children understand what will happen reduces anxiety. The scan uses no needles and causes no pain. The gel applied to the skin feels cool and a little slippery, and the probe creates mild pressure but nothing sharp. Letting a child know these details ahead of time, or even showing them a picture of the equipment, can make the appointment go much more smoothly.
What Happens if You Don’t Prepare Correctly
Skipping the preparation won’t harm you, but it can waste your time. If you eat before an abdominal scan, your gallbladder may have already emptied some of its bile to help digest your food, making it harder to evaluate. Gas from a recent meal can obscure the pancreas entirely. If your bladder isn’t full enough for a pelvic scan, the images may be too unclear to read. In any of these cases, the technician may ask you to wait (and drink more water) or reschedule the appointment altogether.
If you accidentally ate or drank when you shouldn’t have, call the facility before heading in. They can tell you whether it’s worth coming or better to rebook, saving you a trip.