The menstrual cycle—a complex process orchestrated by interactions between many of the body’s tissues, cells, and hormones—reflects a person’s overall health status and can be thought of as a “fifth vital sign,” along with blood pressure, body temperature, heart rate, and respiratory rate. Menstrual irregularities can indicate hormonal imbalances, gynecological diseases, or infections. Stress, changes in weight or diet, certain medications, and other lifestyle factors can cause temporary changes in the menstrual cycle.
Today, there is increasing interest in tracking menstruation, with millions of people using period and ovulation tracking apps to monitor their monthly cycles. In recent months, reports have surfaced in the news and on social media of women experiencing irregular or missing menstrual periods, bleeding that is heavier than usual, and other menstrual changes after receiving COVID-19 vaccines. Thanks to our longstanding commitment to understanding menstruation and endometrial biology, NICHD was poised to rapidly address these anecdotal reports.
Last month, we announced supplemental grant awards to five institutions to explore potential links between COVID-19 vaccination and menstrual changes. Investigators aim to determine whether post-vaccination menstrual changes may be linked to vaccination itself and to explore the underlying mechanisms. For example, immune responses to a COVID-19 vaccine could affect the interplay between immune cells and signals in the uterus, leading to temporary menstrual cycle changes. The research also will account for other factors that may affect menstruation—including pandemic-related stress, lifestyle changes related to the pandemic, and infection with SARS-CoV-2, the virus that causes COVID-19.
Such rigorous scientific studies are critical to provide people with more information about the effects of COVID-19 vaccines on the body and to potentially reduce vaccine hesitancy, and I am proud of NICHD’s role in filling these information gaps. I also see the opportunity to broaden awareness of the types of information that researchers should consider tracking in clinical trials evaluating potential vaccines and therapeutics. Many such studies do not routinely collect data about menstruation and reproductive health, aside from data to exclude pregnancy.
At NICHD’s 2018 “Menstruation: Science and Society” meeting, investigators were challenged to include menstrual status in their research projects and to consider menstruation as a model for understanding other health issues, such as tissue loss and regeneration. Themes from this meeting, highlighted in a paper 
and accompanying editorial in the American Journal of Obstetrics and Gynecology, continue to resonate.
Regularly gathering menstrual cycle data from clinical trial participants would help identify at an early stage the potential effects of vaccines or therapeutics on menstruation, as well as the possible influence of the menstrual cycle on vaccination or treatment outcomes. Further enhancing our understanding of menstruation and how it can be used as a vital sign ultimately will help tailor medical care to individual needs.
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