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. Overall, vaccinated people experienced, on average, about a one-day delay in getting their periods, compared with those who hadn't been vaccinated.

The data, 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.