This review was done because most texts and some recent reviews do not recognize the obstructive cause. It is important only because of the high incidence of acute pyelonephritis during pregnancy. This is so common that it has been given a special name, "pyelitis of pregnancy". We have shown in the monkey that bacteria when introduced into the ureter under low pressure does not often cause pyelonephritis unless associated with ureteral obstruction. The presence of bacteria even with partial ureteral obstruction increases the chances of acute pyelonephritis. Care must thus be taken to eradicate bacteriuria during pregnancy to prevent acute pyelonephritis as has been shown by Kass, especially since vesicoureteral reflux occurs more frequently in the pregnant than nonpregnant primate. When we understand the obstructive cause of hydronephrosis of pregnancy and the effect of positional change, therapy will be more effective. The left lateral position then will not only relieve the pain of acute hydronephrosis of pregnancy but will also greatly assist in the therapy of acute pyelonephritis of pregnancy since it will relieve the associated ureteral obstruction.
