Women who have been vaccinated against COVID-19 had a slight but not clinically significant change in the length of their menstrual cycles compared to unvaccinated women, according to a new study published by researchers from Oregon Health & Science University in Portland in Obstetrics & Gynecology.

Specifically, vaccinated women experienced a less than one-day (0.71-day) unadjusted increase in the length of their menstrual cycle after their first shot and a less than one-day (0.91-day) after their second shot, the study found. 

The increase in cycle length, however, seemed to be driven by several hundred women in the study who received both shots during a single cycle. This subgroup experienced a two-day unadjusted mean cycle length increase. 

“Statistically significant differences existed between vaccination status groups, but the change in cycle length was less than one day, which is below the reportable difference in the menstrual cycle tracking application and is not clinically significant,” the authors stressed.

Reverted to normal

In all cases, the cycles reverted to their original length after two post-vaccine cycles. 

This is the first peer-reviewed research published on the matter through the US National Institute of Child Health and Human Development (NICH), which awarded five grants totaling $1.67 million in October to agencies to explore the link between COVID-19 vaccination and menstruation changes after tens of thousands of women worldwide reported changes in menstruation after receiving the jab. 

“Concerns about a possible association between coronavirus disease 2019 vaccination and abnormal menstrual cycles may lead to vaccine hesitancy,” the study’s authors wrote in their report. “Unfortunately, clinical trials of the current COVID-19 vaccines did not collect menstrual cycle outcomes post vaccine.”

Other NICH-funded research is still underway by teams from Boston University, Harvard Medical School, Johns Hopkins University and Michigan State University.

The study was a retrospective cohort analysis, leveraging data from the Natural Cycles digital fertility-awareness application between October 2020 and September 2021. Nearly 4,000 women – 2,403 vaccinated and 1,556 unvaccinated – between the ages of 18 and 45 who had regular cycles, were not pregnant or taking hormonal contraception were evaluated as part of the study. 

Participants were inoculated with Pfizer (55%), Moderna (35%) or Johnson and Johnson (7%). 

Six cycles were analysed for each participant – for those who were vaccinated, three cycles before the first shot and three cycles during and after.

Study limitations

Dr Itamar Netzer, a gynaecologist and a sub-district director for Clalit Healthcare Services in Israel, there were some challenges with the study, which were also noted by the research team. These included that it relied completely on Natural Cycles data and application users tend to be mostly white, college-educated and have lower BMIs than the average American woman. 

In addition, he said that because the study relies on self-reported data by women, it is possible that these women did not accurately record their number of bleeding days. 

“Women with underlying gynaecological disorders may experience greater differences,” according to Netzer, who noted that only healthy women were included in the study.

Menstrual cycle timing is regulated by the hypothalamic-pituitary-ovarian axis, the researchers explained, which can be affected by life, environment and health stressors. They hypothesised that because mRNA vaccines create a robust immune response or stressor this could temporarily affect this axis, which could be the cause of the slight shift in cycle length. 

Netzer said that the research shows that the vaccine is “probably safe for everyone” and especially menstruating women. He noted that separate studies have shown that the vaccine is safe for pregnant women and does not impact the rate of stillbirths or premature delivery.

“In contrast, we have seen the hazards of COVID-19 on pregnancy,” Netzer stressed. “A pregnant woman who catches COVID is two to five times more likely to be hooked up to a lung-heart machine than a non-pregnant woman.

“We are quite sure the vaccine is safe in pregnancy,” Netzer continued, “and we know the [COVID-19] disease is dangerous for pregnancy.”