Not long after the rollout of coronavirus vaccines last year, women around the country began posting on social media about what they believed was a strange side effect: changes to their periods. Now, new research shows that many of the complaints were valid. A study of nearly 20,000 people around the world shows that getting vaccinated against covid can change the timing of the menstrual cycle. Vaccinated people experienced, on average, about a one-day delay in getting their periods, compared with those who hadn't been vaccinated. The data for the study, published Tuesday in the British Medical Journal, was taken from a popular period-tracking app called Natural Cycles and included people from around the world, but most were from North America, Britain and Europe. The researchers used "de-identified" data from the app to compare menstrual cycles among 14,936 participants who were vaccinated and 4,686 who were not. The data showed that vaccinated people got their periods 0.71 days late, on average, after the first dose of vaccine. However, people who received two vaccinations within one menstrual cycle experienced greater disruptions. In this group, the average increase in cycle length was four days, and 13 percent experienced a delay of eight days or more. Many people on social media have complained of longer, heavier and more-painful periods after getting vaccinated. Preliminary findings from a different study suggest that getting a coronavirus vaccine sometimes may cause heavier periods.