Postoperative maternal TAC , TOS and OSI levels significantly increased in the oxygen group ( p = 0.047 , < 0.001 and 0.038 , respectively ) ; umbilical artery TAC levels significantly increased in the oxygen group ( p = 0.003 ) ; and umbilical artery TOS and OSI levels significantly increased in the air group ( p = 0.02 and < 0.001 , respectively ) .