Between patients taking pregnancy tests and those that have drank a lot of water in preparation for an ultrasound, busy pregnancy clinics can often run into the problem of not having enough bathrooms. A recent discussion of this problem lead me to an interview with Dr. Susan Rutherford, an OB/GYN and Maternal-Fetal Medicine specialist. According to Dr. Rutherford, patients should not come in with full bladders- though still very prevalent in doctors offices across the country, this advice is outdated. Although this was helpful when using old radiology equipment, newer ultrasound machines make this discomfort unnecessary.
This is a paraphrase of her explanation:
Old sonography equipment used linear (flat) array transducers, which send sound waves only in parallel lines. Since the non-pregnant and first trimester uterus is in the pelvis, you need to angle the sound waves from the abdomen down into the pelvis in order to see around the symphysis pubis. This was impossible with the linear transducer, so patients needed to have a full bladder in order to lift the uterus into the abdominal cavity. Contemporary curvilinear transducers fan out sound waves, and are able to capture images behind the symphysis.
A full bladder is also an ‘acoustic window’ full of water that facilitates the transmission of sound waves, as opposed to bowel or other tissue that reflects waves more. Thus, theoretically you can see the uterus more clearly if the sound waves are sent through the full bladder first. However, it rarely helps to see the pregnancy itself within the uterus. If you cannot see the pregnancy clearly with an empty bladder, you will not see it better with a full bladder- it is better to move to transvaginal and skip the misery of a full bladder.
Although Dr. Rutherford has never noticed fluid in the bladder improving her images, she added that those who really feel it helps do not need to have patients purposefully fill their bladders. Enough of an acoustic window can be created within a few minutes after drinking, and most women have enough as they are when they arrive. Additionally, if patients arrive and really feel like voiding, it is alright to let them. The tools we have are sufficient, especially when you include the transvaginal probe.