This is a multicenter, randomized, double-masked, controlled, phase III study of repeated use of C31G vaginal gel compared to Conceptrol® Vaginal Gel as the primary method of contraception over six months (183 days) and at least six cycles of use. In addition, there is an opportunity for subjects to continue with study treatment for up to twelve months (365 days) and twelve cycles of treatment upon completion of the first six months of treatment.