The changing sonographic appearance of fetal kidneys during pregnancy.

One hundred consecutive women between 11 and 40 weeks of gestation were studied to evaluate the changing appearances of the fetal kidneys. In the first trimester, the kidney was never positively identified. Between 15 and 26 weeks it was seen but was difficult to distinguish from surrounding tissue. In the early third trimester, either an echogenic border or increased echogenicity of the renal sinus was observed; in the late third trimester this increased echogenicity was observed in both areas. It is thought that the echogenicity is the result of fat deposition in the pararenal space and in the renal sinus.