Purpose: To show evidence that progesterone therapy is useful in preventing miscarriages in women who are more prone to having them. Methods: Vaginal progesterone therapy was evaluated in women with a previous history of miscarriage or in women with infertility related to luteal phase defects. Results: The results favor the benefit of using progesterone to diminish the risk of miscarriage. Other methods of stimulating progesterone production, e.g., human chorionic injections, are also effective. Conclusions: Progesterone therapy, especially when given vaginally, is effective with few side-effects and is safe. Thus the evidence suggests that one should err on the side of over-treatment rather than under-treatment in certain circumstances, e.g., advanced woman’s age, previous history of miscarriage, or the use of follicle maturing drugs.