Vaginal defence is lost:

Vaginal pH: newborn infant -4.5 to 7 pubey -alkaline to acidic reproductive age -4 to 5.5 pregnancy -3.5 to 4.5 post menopausal -6 to 8 Acidity of vagina is because of doderlein's bacilli. It is a rod-shaped gram-positive bacillus which grows anaerobically on acid media. It appears in the vagina 3-4 days after bih and disappears after 10-14 days. It appears again at pubey and disappears after menopause. Its function is to conve glycogen present in vaginal mucosa into lactic acid so that the vaginal PH is maintained towards acidic side, which prevents other pathogenic organisms from invading the vagina. Ref: D.C Dutta's Textbook of GynaecologyPg no:7,6th edition