Following widespread concern that Covid-19 vaccines were interfering with menstrual cycles, a new study discovered that the inoculations cause periods to be slightly and temporarily delayed.
According to research funded by the US government and published in the medical journal Obstetrics & Gynecology, women who had been vaccinated experienced an increase in their usual menstrual pattern of just under a day.
The study comes at a time when some women have reported irregular cycles, as well as heavier and sometimes painful bleeding. It was discovered that those who received both vaccine doses during the same cycle had their periods two days later than usual.
“These variances appear to resolve quickly, possibly as soon as the next cycle after vaccination,” lead author Alison Edelman, a professor of obstetrics and gynecology at the Oregon Health and Science University in Portland, said in a statement.
“Our findings are reassuring. On a population level, the changes we are finding indicate no cause for concern for long-term physical or reproductive health.”
The study’s publication may help to alleviate some of the anti-vaccine hysteria that has long circulated on social media. It’s still unclear why Covid vaccinations affect those cycles, which vary greatly for many people over their lifetime but typically last around 28 days for the majority.
The researchers examined anonymized fertility app data from just under 4,000 vaccinated and unvaccinated 18 to 45-year-old U.S. resident women between October 2020 and September 2021.
The researchers studied data collected from three consecutive cycles before and three consecutive cycles after the 2,403 people were vaccinated with American-approved shots. Six consecutive menstrual cycles were examined in the remaining unvaccinated cohort.
While approximately 10% of the inoculated women experienced fairly large and clinically significant changes in menstrual length of 8 days or more, the researchers stated that their periods returned to normal within two post-vaccine cycles. There were no changes in menstrual length in either group, and the study’s authors stated that their findings cannot be explained by pandemic-induced stress because the unvaccinated group saw no changes over a similar time period.
However, questions remain about the effect of vaccines on other menstrual symptoms, such as bleeding, and the researchers acknowledged that the study had several limitations, such as the inclusion of women who did not use hormonal contraception.
Another factor was that they chose people who had consistent normal period lengths. They were more likely to be white, college-educated, and have a lower body mass index than the average American, and thus were not nationally representative.