A cross-sectional study of changes in fetal renal size with gestation in appropriate- and small-for-gestational-age fetuses.

This cross-sectional study aimed to determine whether there are differences in kidney size and shape in small- and appropriate-for-gestational-age fetuses at different gestations. In the Fetal Growth Clinic of a large university hospital in England, 219 singleton fetuses were studied cross-sectionally at different gestational ages from 22 to 38 weeks. At each gestational age, the fetal kidney length, circumference and anterior-posterior and transverse diameters were measured using an Aloka 650 SL machine. At birth, the fetuses were classified as small or appropriate for gestational age, depending on the birth weight centile; the various kidney measurements for the two groups were compared. The kidney lengths at different gestational ages were similar in the two groups. The circumference and transverse and anterior-posterior diameters were significantly greater in the appropriate-for-gestational-age fetuses from about 28 weeks' gestation. We conclude that differences in fetal kidney size with gestation manifest from as early as 26-28 weeks. The differentially smaller anterior-posterior and transverse kidney diameters in small-for-gestational age fetuses, resulted in altered renal morphology, producing sausage-shaped kidneys.