Until 1993, women weren’t included in trials for new drugs. “The thought was women might get pregnant during a study, and that could be harmful to the fetus,” says Dr. Janine Clayton, associate director for women’s health research at the National Institutes of Health (NIH). Even after the FDA lifted the ban, most studies continued to use male participants. This bias ran further down the pipeline: Historically researchers have used male animals in preclinical trials to avoid having to control for estrus cycles in female animals.