Development of Urine Production and Urination in the Human Fetus Assessed by Real‐Time Ultrasound

To evaluate the physiological profile of fetal urine production with advance in gesta‐tional age, we observed the volume of urine retained in the bladder, using real‐time ultrasound. Here, the parameters of maximum urine volume, periodicity of urination and the urine production rate were analyzed. Urine volume can be measured by 3 orthogonal diameters of the bladder, provided that the stereognostic image of the bladder is assumed to be an ellipsoidal configuration. Investigated were 1,261 normal fetuses between 12 and 42 weeks of gestation. Three critical ages were noted, with respect to development of urine production and urination: 12, 18–20, and approximately 37–38 weeks of gestation. The first and the second indicate the onset of urine production and the start of alternating changes in urinary retention and urination, respectively. From the third period toward the end of the pregnancy, the urine production rate begins to decline.