Hyperplasia is an exaggerated swelling reaction of the vaginal tissue to estrogen during the proestrus and estrus phases of heat. Should the swollen vaginal tissue become too swollen and no longer be contained within the vagina, it may bulge out, appearing as a tongue shaped mass, through the vulva, which is the external female genital organ. The mass may prevent the dog from mating and affects mostly young female dogs of the larger breeds that have not been spayed. Vaginal hyperplasia is thought to be mainly caused by estrogen stimulation as well as dogs having a genetic predisposition towards having this condition. Severe hyperplasia may lead to prolapse.