It is universally observed in women of all ethnicities that a full-term pregnancy has a protective effect. This phenomenon is not limited to humans, having also been observed in rats and mice. The aim of this study was to examine the effect of various natural and/or synthetic estrogens and progestins at different doses and durations on the incidence of mammary cancer in mice following exposure to N-methyl-N-nitrosourea, a known carcinogen. Estriol, which is known to increase during pregnancy, was tested alone and in combination with progesterone. Synthetic estrogens and progestins in various combinations were also tested. All hormone treatments were given 2 weeks after administration of the carcinogen. Researchers found that treatment with 30mg estriol and 30mg progesterone for a 3-week period significantly reduced the incidence of mammary cancer.