Measuring the fetal kidney with ultrasonography

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Abstract
To establish a normal range of measurements for the external and internal dimensions of the fetal kidney and, if possible, to correlate these measurements with gestational age. The external and internal dimensions of the fetal kidney were measured in 810 women selected on the grounds of obstetric, fetal or technical risk factors from a consecutive series of 1136 women attending the ante-natal ultrasonography clinic of a district general hospital. Post-natal data were obtained from medical records on 1122 newborns. The maximum of pairs of renal measurements was used for analysis. Measurements were cross-sectional in 347 cases and longitudinal in 463. The total number of measurement episodes was 2294. External renal dimension and gestational age were closely related, enabling accurate growth centile charts to be constructed. The correlation between renal pelvic dimension and gestational age was weak. The maximum dimension of the renal pelvis at any gestational age in 92.7% of fetuses was < 5 mm. Using longitudinal measurements, 6.5% of cases with a renal pelvic dimension of < 5 mm at the first scan measured > or = 5 mm at a subsequent scan, but 53.3% with a dimension of > or = 5 mm at the first scan also measured > or = 5 mm at the subsequent scan. It is possible to assess the growth and size of the fetal kidney according to gestational age but this does not apply to the renal pelvis. A renal pelvic dimension of > 5 mm at any gestational age is unusual and dilatation beyond this level should prompt a detailed post-natal urological investigation.