While an evaluation of the estrous cycle in laboratory rodents can be a useful measure of the integrity of the hypothalamic-pituitary-ovarian reproductive axis, it can also serve as a way of insuring that animals exhibiting abnormal cycling patterns are disincluded from a study prior to exposure to a test compound.
The Rodent Estrous Cycle: Characterization of Vaginal Cytology and Its Utility in Toxicological Studies
Birth Defects Research Part B: Developmental and Reproductive Toxicology, 2007