Covid vaccination linked to slight increase in menstrual cycle, NIH study confirms

Covid-19 vaccination has been linked to a slight increase in the length of a woman’s menstrual cycle, delaying the onset of bleeding by a few hours, according to a large international study funded by the National Institutes of Health.

dr. Diana Bianchi, chief of the NIH Institute for Child Health and Human Development, said the changes after vaccination seem small, temporary and within the normal range. However, the longer menstrual cycle, usually about a month in length, did not necessarily lead to an increase in the number of days of bleeding, according to the health authority.

A change in menstrual cycle length of eight days or less is considered within the normal range of variation, NIH said. According to the study’s findings, the participants’ menstrual cycles increased by an average of 0.71 days, or less than 24 hours, after the first vaccine dose and by just over half a day after the second dose. Women who received both vaccine doses in a single menstrual period saw their cycles increase by 3.91 days.

But more than 1,300 women saw their cycles increase by eight days or more, representing 6.2% of the vaccinated individuals and 5% of the unvaccinated people in the study. Younger women who had longer cycles before vaccination were more likely to experience a greater delay in the onset of their periods.

After the vaccination series was completed, cycle lengths had largely returned to normal for women who received one dose per menstrual cycle and by about 20 hours for those who received both doses in a single cycle.

Nearly 20,000 people took part in the study in Canada, the UK, the US, Europe and other parts of the world. Participants received one of nine different vaccines: Pfizer-BioNTech, Moderna, Johnson & Johnson, AstraZeneca, Covishield, Sputnik, Covaxin, Sinopharm, and Sinovac.

The changes in menstrual cycle length did not differ between vaccines.

The researchers used data from a fertility tracking app called Natural Cycles. Women provided information about their temperature and menstrual cycle to the app. Users of the app can select an option to provide their data for research purposes without personally identifiable information.

The researchers released preliminary findings in January suggesting a link between Covid vaccination and a longer menstrual cycle, and the study published this week confirmed the link. NIH provided $1.67 million to five research institutions to investigate the problem.