Over 40% of people with regular menstruation cycles reported heavier periods after receiving COVID-19 vaccinations, according to research published Friday.

The web-based survey of more than 39,000 respondents found that 42% bled more heavily than usual after vaccination, while 44% reported no changes.

The study, published in the journal Science Advances, comes after many people who menstruate shared their experiences of heavier or unexpected menstrual bleeding after receiving COVID-19 jabs.

Who is more likely to have menstrual bleeding changes after vaccination?

Of those surveyed, 90% identified as women, and 9.1% as gender-diverse.

Women with conditions such as endometriosis and polycystic ovary syndrome were more likely to experience heavy bleeding.

Respondents who typically did not menstruate also reported breakthrough bleeding. This included 71% of people on long-acting reversible contraceptives, 39% of people on gender-affirming hormones and 66% of people who were postmenopausal.

As the study is survey-based, it might not represent the population as a whole, Victoria Male, a researcher in reproductive immunology at Imperial College London, who was not involved in the study, told DW.

"We might expect people who noticed a change to be more motivated to participate in the survey. But the findings are useful — they tell us what kinds of people might be more likely to experience heavier than normal bleeding," Male said.

Should I be worried about my reproductive health after receiving a COVID vaccine?

Earlier this year, there were claims that the COVID vaccines caused women to have irregular menstrual cycles for longer.

These claims originated from US research showing that COVID-19 vaccination delayed menstrual bleeding.

The study found, however, that people's periods were only one day late and the length of the period remained largely unchanged. The changes were also found to be temporary.

Another study by the Norwegian Institute of Public Health (FHI) suggested that some women had heavier periods after being inoculated.

"Unfortunately, both vaccine-hesitant and anti-vaccine individuals and organizations conflated the possibility of short-term menstrual changes with long-term harms to infertility," the study authors wrote.

Such a minimal change in the menstrual cycle is not significant, according to the International Federation of Gynecology and Obstetrics (FIGO), a worldwide nongovernment organization representing obstetricians and gynecologists. FIGO classifies changes of less than eight days as normal.

Though the majority of menstrual changes after vaccination are temporary, it is still important to take the side effects seriously, Katherine Lee, the lead study author and a biological anthropologist at Washington University School of Medicine in the US, told DW.

"People become concerned when they notice their body is doing something unusual from their individual norms," Lee said.

"The vast majority of people experience menstrual changes for a short time, but, if you are concerned about what you are experiencing, you should talk to your doctor," Lee said.

Why do coronavirus vaccines increase menstrual bleeding?

A person's monthly cycle is controlled by hormones released from the hypothalamus, the pituitary gland and the ovaries. This prepares the female body for implantation of a fertilized egg.

Menstrual cycles are easily altered by hormonal changes. The uterine reproductive system is adaptable and has evolved to weather short-term challenges to leave long-term fertility intact.

Hormonal changes caused by psychosocial stress, strenuous exercise (such as running marathons) and fasting commonly cause changes to menstruation.

Lee said the link between vaccination and heavier bleeding was likely due to the immune responses caused by the vaccine, specifically inflammation caused by cytokines ⁠— small proteins that tell the immune system when it needs to do its job.

Lee said the endometrium — the lining of the uterus — needed to bleed and clot appropriately as it repairs and heals.

"A disruption of immune function or inflammation is going to disrupt those processes," Lee said.

Transparency about periods and vaccine side effects

Part of the motivation for the study was to raise awareness of people's health concerns, the authors said.

"People just don't tend to talk about periods in public very much," Lee said. "We think that transparency is really important, and that people understand a short-term side effect, like increased menstrual bleeding, can occur as part of a safe vaccine."

Talking openly helps reduce stigma because people don't feel alone. Transparency and starting from a place of believing what people say about their bodies is so important to increase trust, Lee continued.

Male agreed, saying many people aren't aware of the normal fluctuations in menstruation.

"This has two effects," she said. "First, sometimes we worry about normal variation. Second, sometimes people are having periods that they should get checked out, but they don't realize."

Edited by: Louisa Wright