The menstrual cycle is a vital sign across the lifespan.

The menstrual cycle is an underutilized but powerful tool for understanding gynecological and general health across the lifespan. However, menstrual-related concerns are often underdiagnosed or ignored. Reasons for the underdiagnosis and neglect of menstrual disorders are many, but include that menstrual cycle characteristics are not routinely assessed or documented in clinical practice. Although the menstrual cycle has been recognized as a vital sign in adolescence, this position has not been advanced for adults. In this Viewpoint, we argue that the menstrual cycle functions as a vital sign across the lifespan, providing critical information that can be used to guide and monitor clinical treatment and symptom management, and inform screening and preventive care. The menstrual cycle can serve as a health indicator and cycle characteristics can predict chronic disease later in life. We also illustrate the value of collecting menstrual cycle data to research across disciplines. Documenting the menstrual cycle as a vital sign in both clinical and research contexts has the potential to profoundly improve patient well-being, clinical care, and public health.