Pyelonephritis is bacterial infection of a kidney, which usually arises as the result of a urinary tract infection. During pregnancy it more commonly affects the right kidney, because the uterus lies over to the right and may obstruct urinary drainage. Besides symptoms of urinary frequency and pain, there may be back pain together with fever and nausea and vomiting. This can predispose to the development of premature labour and, in early pregnancy, hyperemesis gravidarum - severe vomiting (described in this section). While mild symptoms of cystitis can be neutralised by taking medicines to change the amount of acidity in the urine, it is important that persistent symptoms are investigated in order to rule out urinary tract infection and prevent the development of pyelonephritis. Taking lots of fluid will help prevent problems.