Researchers documented the impact of COVID-19 vaccinations on the menstrual cycle, and found it may lead to cycle length increases. 

This comes after countless women have made claims on social media that COVID-19 vaccines disrupted their menstrual cycle. The National Institute of Health (NIH) awarded $1.67 million to five U.S. research institutions across the country to study how the COVID-19 vaccine affects menstrual cycle patterns. 

Last week, the journal Obstetrics and Gynecology published a study by researcher Alison Edelman, MD, of Oregon Health & Science University (OHSU), Portland, and colleagues that documented these experiences and found temporary changes in menstural cycle length.

Menstrual cycle timing is controlled by the the hypothalamus in the brain, which causes the near by pituitary gland to produce chemicals that enables ovaries to produce the sex hormones oestrogen and progesterone. Changes in the  body such as vaccines and infections can cause disruptions in hormone signals. 

Edelman and co-authors added the trial findings “support and help explain the self-reports of changes in cycle length.”

Researchers collected and analyzed menstrual cycle data of nearly 4,000 consenting users between 18 and 45 years of age from a fertility tracking app, "Natural Cycles”, between October 2020 through September 2021. All participants were either three cycles post-pregnancy or post-use of hormonal contraception. 

A total of  3,959 subjects participated, and of that, 2,403 were vaccinated and 1,556 were unvaccinated for six consecutive menstrual cycles. Researchers analyzed three cycles prior to the vaccine and post-vaccine, or six cycles for those who remained unvaccinated.

Data from vaccinated and unvaccinated populations were compared to determine cycle length changes. They found vaccine recipients experienced a cycle increase of 0.71 days in their menstrual cycle after their first dose, and those who received their second dose had a slightly longer increase at 0.91 days. Unvaccinated individuals saw no significant changes. 

A subgroup of  358 vaccine recipients who received both vaccine doses in the same menstrual cycle showed a larger increase in cycle length of two days. 

Researchers note that their findings may not be generalized considering those using the Natural Cycles app were mostly White, college educated, with lower BMIs than the general population and were also not using contraception with normal menstrual cycle lengths. Additionally, the study did not include data on COVID-19 infections for either subject groups. 

Edelman and co-authors noted that additional research is needed to determine how COVID-19 vaccination could potentially influence other menstrual characteristics such as associated symptoms and characteristics of bleeding.

In a statement, the National Institute of Child Health and Human Development director Diana W. Bianchi, MD, said  the results from the published study are “reassuring". She added that little research has previously been conducted on how vaccines for COVID-19 or vaccines for other diseases could potentially influence the menstrual cycle. 

“It is reassuring that the study found only a small, temporary menstrual change in women,” she noted. “These results provide, for the first time, an opportunity to counsel women about what to expect from COVID-19 vaccination so they can plan accordingly,” Dr.Bianchi said. 

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