My wife done tiffa scan at 20 th week, doctor told baby kidney is 3.7 mm. is that normal?

No: The normal length of a kidney in a 20 wk fetus should be 2.1 cm (21 mm). I trust the other kidney was more normal in size? It is possible that the small kidney may not have formed and the adrenal gland was measured. Sometimes a kidney may form in the pelvis or on the same side as the other kidney. The fetus may require a repeat scan later on to further evaluate the situation.