The thickness and health of the squamous epithelium of the vagina is strongly influenced by the presence of oestrogen during puberty, the reproductive period and particularly during pregnancy. At maturation the flaking off of dead cells at the surface and the subsequent release of glycogen from these cells is the power supply of Döderlein lactobacilli, which converts it into lactic acid and creates a low (acid) pH between 4 and 4.5. In children and after the menopause the pH is higher than 4.7, but during the fertile period it falls to less than 4.5 in healthy conditions.