The normal pattern of renal development late in gestation in rats involves a transient variation in growth rates. While the renal papilla is slowly and steadily increasing in length the renal parenchyma is rapidly and almost exponentially increasing in weight. This disparity in growth rates frequently results in a kidney that has an enlarged renal pelvis late in gestation; but this is only an “apparent hydronephrosis” since it disappears shortly after birth. Methyl salicylate retarded renal development, particularly growth of the renal papilla, and increased the number of both transient and permanent renal abnormalities in rat fetuses near term. Preliminary evidence suggested that if the renal papilla has reached a particular stage of development 1 day before birth, and is not accompanied by grossly apparent renal malformations, the kidney will be normal at weaning. However, at birth, kidneys with an absent renal papilla, but that appeared otherwise normal, cannot be differentiated on a gross anatomical level from those that are destined to be permanently abnormal.
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Woo et al. (1972) studied this question.