Researchers exploring whether Covid vaccines may disrupt menstrual cycles have said any potential changes to periods are short-term and do not affect fertility. Though any link between the jab and changes in periods is yet to be proven, anecdotal accounts have been highlighted in blogs and on social media. Speaking to the Guardian’s Science Weekly podcast, Dr Kate Clancy, an associate professor at the University of Illinois, and Dr Katharine Lee, a postdoctoral researcher at Washington University School of Medicine, said they both experienced temporary changes to their menstrual cycles after receiving Covid vaccinations, leading them to start a survey to explore whether others had similar stories. The results, they say, suggest the experience is highly variable. “[Among those reporting changes] people on long-acting hormonal contraceptives, people on gender-affirming hormones and post-menopausal people were all reporting effectively surprise periods or breakthrough bleeding,” said Clancy. “And then of people who currently have periods, some of them were reporting earlier and heavier periods – but I will also say that some were also reporting later and lighter periods. And there definitely is a very large number of people who have reported that they have really experienced no changes at all.” Clancy, whose research focuses on understanding the impact of environmental stresses on menstrual cycles, added that the changes did not appear to affect a large number of people, or last for long. “This is not a universal experience,” she said. “I would say this also appears to be something that is very short-lived – it is one to two cycles, max, of changes.” However, the potential link is not clearcut: many people experience variations to their cycles from month to month, and patterns of menstruation can be influenced by factors including stress. What is more, because of the nature of the pandemic, researchers cannot now track people’s periods before and after vaccination in a controlled way – although Clancy notes that would have been possible during the vaccine trials. Dr Sue Ward, a vice-president at the Royal College of Obstetricians and Gynaecologists (RCOG), said something as all-consuming and life-changing as a global pandemic could result in women experiencing their periods differently. But she added: “Anecdotally some women seem to be reporting heavier periods after receiving the Covid-19 vaccine and we would support more data collection in this area to understand why this might be the case.” Dr Pat O’Brien, another RCOG vice-president, agreed. “Many women will experience a temporary change in their periods from time to time during their lives. And right now, many women in their 30s are having the Covid vaccine. So it seems inevitable that in some women these two events will coincide by chance,” he said. “If, however, these changes persist, or you have any new vaginal bleeding after the menopause, you should see your doctor.” Clancy said there were potential mechanisms by which Covid vaccination may disrupt menstruation, among them that the jabs may trigger a large general inflammatory response. “The uterus and the ovaries are constantly doing tissue remodelling,” she said, noting that this included ovulation and the growth and repair of the lining of the uterus. “These are all things that are communicated through inflammatory processes. And so you can imagine if you have a giant wallop of an immune event, then all of the inflammation that goes with that might somehow also affect many other systems of the body kind of unintentionally.” Clancy added it was not a bad thing. “It’s more just a ‘Hey, this is this is how we know it’s really doing its job’,” she said. O’Brien said it was important not to confuse menstrual cycles with fertility and the ability to have children: “There is no evidence to suggest that Covid-19 vaccines will affect fertility.” Lee agreed. “What this vaccine could do is potentially slightly shift things like timing of bleeding or timing of ovulation, or some of these things for those one or two cycles. So for that one or two cycles, it is possible that if you are trying to become pregnant, it might not line up in timing the way you expect it to,” she said. She also noted that evidence showed there was no difference in miscarriage rates for people who are vaccinated and those who are not. “We do know that contracting Covid is very, very bad for the pregnant mother and the baby,” she said. “So getting vaccinated provides huge protection against getting Covid and potentially getting really sick in a way that could harm both the parent and the child.”