The purpose of our study is to see whether vaginal and cervical pH are helpful to screen for bacterial vaginosis and cervicitis during pregnancy. One hundred and seven pregnant women underwent prospectively both vaginal pH and cervical pH measurement and screening for microbial infections of the lower genital tracts at 10, 20 and 30 gestational week between February 1993 and August 1993. The value of vaginal pH significantly elevated in bacterial vaginosis (P < 0.05). Vaginal pH values also significantly elevated in patients who had had sexual intercourse 2 days before the dates of examinations (P < 0.01). Cervical pH value exhibited no significant change in bacterial vaginosis and cervicitis. Vaginal pH is a valid indicator for screening of bacterial vaginosis during pregnancy. However, vaginal pH might be influenced by the presence of semen. Cervical pH is not a useful parameter for screening of cervicitis during pregnancy.