Hourly fetal urine production rate in the near-term fetus: is it really increased during fetal quiet sleep?

Fetal bladder volume and hourly fetal urine production (HFUPR) is calculated on the assumption that the fetal bladder is ellipsoid in shape. A recent validation study demonstrated a progressive overestimation at increasing bladder volumes. This may be due to changes in shape of the fetal bladder at increasing volumes. Two independent papers have shown increased HFUPR during fetal behavioural state 1F (S1F) when compared with S2F. The aim of the present study was to assess whether this increase of HFUPR during S1F, previously observed by others, could be the result of an error introduced by the method of volume calculation. A retrospective evaluation was performed in a series of 208 HFUPR measurements in 123 normal near term pregnant women attending a low-risk atenatal clinic. Adequate bladder filling in both states was identified in 43 recordings. Maximum fetal bladder volumes were greater (> 10 ml) during S1F in comparison to S2F in 56% of these recordings and HFUPR was significantly greater during S1F only in these cases. Bladder volumes are usually lower during S2F as a result of fetal voiding, which occurred in association with 22 of 36 transitions from S1F to S2F, and only 1 of 13 transitions from S2F to S1F (P < 0.001). When disregarding calculated bladder volumes in excess of 20 ml for the purpose of calculating HFUPR, eleven recordings remained. HFUPR calculated in this way was significantly lower in comparison to measurements where larger bladder volumes were included and no difference was observed between states. This implies that the differences observed are the result of the greater error in calculating bladder volumes and HFUPR during S1F, where volumes are usually greater and that calculation of fetal bladder volume should not be performed on the assumption that the bladder is ellipsoid in shape. Alternative techniques include limiting measurements to a maximum volume of approximately 20 ml, when the bladder is usually ellipsoid in shape or basing volume calculation on the surface area of a series of sagittal views as suggested by Hedriana and Moore [Hedriana HL, Moore TR. Ultrasonographic evaluation of human fetal urinary flow rate: accuracy of bladder volume estimations. Am J Obstet Gynecol 1994;170:1250-1254; Hedriana HL, Moore TR. Accuracy limits of ultrasonographic estimation of fetal urinary flow rate.