The fetal urinary bladder, also known as the bladder bubble, is visible as an anechoic cyst on ultrasound in most fetuses from 12-13 weeks of gestation 1.
The size of the bladder is denoted by measuring the longitudinal bladder diameter from superiorly to inferiorly in the sagittal plane. Age-related nomograms can be used for accurate size measurement. However, the bladder bubble should not exceed 7 mm, 30 mm and 60 mm in the first, second and third trimesters respectively 2. Also, fetal urinary bladder wall thickness should not exceed 2 mm throughout pregnancy 3,4.
An adequately filled urinary bladder is a good measure of kidney function. After the first trimester, fetal urine significantly contributes to amniotic fluid volume, which is also essential for the normal development of the lungs. So, the amniotic fluid volume should also be assessed along with fetal bladder volume.
Radiographic features
Ultrasound
The sagittal plane is ideal for the measurement of bladder diameter. The coronal section shows the stomach and the bladder bubble in the same plane. The visualization of a small bladder in the first trimester can be aided by color Doppler, which highlights the umbilical arteries at both flanks of the urinary bladder.
The urinary bladder empties and refills every 25-30 minutes during the second and third trimester and more frequently in the first trimester 5. So, if the bladder bubble is not visualized, the study may be repeated after an adequate interval.
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