In a female fetus, the uterus starts out as two small tubes. As the fetus develops, the tubes normally join to create one larger, hollow organ the uterus. Sometimes, however, the tubes don't join completely. Instead, each one develops into a separate structure. This condition is called double uterus (uterus didelphys). A double uterus may have one opening (cervix) into one vagina, or each uterine cavity may have a cervix. There may even be two vaginas.