Kidney dysplasia: a condition in which the internal structures of one or both of kidneys do not form normally during fetal development. As a result, urine collects inside the affected kidney and forms fluid-filled sacs (cysts). The cysts replace normal kidney tissue and prevent the kidney from functioning. Kidney dysplasia can affect one kidney or both kidneys. A fetus with severe kidney dysplasia affecting both kidneys typically will not survive birth. Babies with kidney dysplasia in only one kidney may not show any symptoms. Kidney dysplasia can be caused by genetic conditions or the use of certain prescriptions or drugs by the mother during pregnancy.