Fetal renal to abdominal ratio: A constant measurement throughout gestation

BACKGROUND: Sonographic assessment of the fetal kidneys is an integral part of the prenatal anatomical survey. OBJECTIVES: To evaluate the fetal renal to abdominal (RTA) ratio throughout pregnancy and to investigate whether this ratio can be a potential diagnostic landmark for congenital anomalies of the kidney and urinary tract (CAKUT). METHODS: Measurements of the anterior-posterior diameters of the fetal kidney and fetal abdomen (APAD) were obtained prospectively. The RTA was calculated as the ratio between them in in two groups: normal population vs. CAKUT cases. RTA in CAKUT cases was compared to RTA in a normal population. RESULTS: The study group was comprised of 210 women. The mean gestational age for the fetuses was 31 ± 5.6 weeks (range 14-40 weeks). Fetal RTA ratio was found to be 0.28 ± 0.03 throughout pregnancy from early second trimester to term, with high reproducibility of measurements. During the study period the RTA was evaluated in nine cases referred for suspected CAKUT. All cases demonstrated a different ratio according to the renal anomaly. High ratio was observed in one case of overgrowth syndrome (Beckwith Wiedenmann syndrome; 0.47), three cases of infantile polycystic kidney (0.45-0.47), and three cases of a solitary kidney (0.31-0.35), while cases of dysplastic kidneys revealed a low ratio (0.14-0.18). CONCLUSIONS: Prenatal RTA ratio is constant throughout gestation. An abnormal ratio should lead to meticulous renal investigation to rule out kidney disease.