Vaginal physiology during menstruation.

We studied 18 young healthy women on the second, fourth, and 14th day of their menstrual cycle. Vaginal fluid was collected for measurement of oxygen and carbon dioxide tension (PO2 and PCO2) and specimens were collected for bacteriologic examination. The vaginal pH was measured at four different sites and the redox potential was measured in the top of the vagina. Staphylococcus aureus was found in three women. The PO2 ranged from 0 to 77 mm Hg on day 2; 0 to 76 mm Hg on day 4; and 0 to 53 mm Hg on day 14. The mean PCO2 (+/- SE) was 46 +/- 2 mm Hg on day 2; 62 +/- 4.5 mm Hg on day 4; and 50.6 +/- 8.5 mm Hg on day 14. The mean vaginal pH (+/- SE) was significantly higher on day 2 (6.6 +/- 0.3) compared with day 4 (5.3 +/- 0.3) and day 14 (4.2 +/- 0.2). The redox potential was significantly higher on day 14 compared with day 2 and day 4. No differences were found in values of women who took birth control pills and those of the women who did not.