Changes to your period after getting COVID vaccine are short-lived, new research suggests

People who had slight changes in their menstrual cycle after getting the COVID-19 vaccine only experienced those changes for a brief time period, as a new study "reassures" there is little risk in fertile individuals getting inoculated.

Last week, a study funded by the National Institutes of Health found the vaccine did not affect the chances of people trying to conceive a baby. However, getting the virus itself appeared to slightly lower the chances of conception, likely due to fevers reducing sperm count.

Now in a new study published in the journal The BMJ on Wednesday, nearly 10,000 people who have periods observed in the United States and Norway showed some experienced slight changes, such as heavier flow or delay, within their first two menstrual cycles following the vaccine before things went back to normal.

Victoria Male, a reproductive specialist at Imperial College London and lead author of the study, said people have become more aware of changes to their periods shortly after getting vaccinated, but there was little known if it was a common occurrence or people were just more aware of their periods.

"The main reason it would be good to know if these changes are associated with vaccination are practical: people need to plan for their periods, so it would be good if we could let them know if they might expect a change one month," Male said in an email to USA TODAY. "People also need to know if vaccination might cause a delay to their period so they don’t start worrying – or celebrating – that they are pregnant if this happens to them."

The first observation was done on 3,959 Americans who had logged at least six straight menstrual cycles on a tracking app, with 2,403 of users vaccinated. Findings showed the first dose of the vaccine had no effect, while the second dose had an average delay of 0.45 days.

However, some who got both doses of the vaccine in the same cycle had longer delays, with 358 experiencing a two-day delay. Of the 358 people, 39 had a delay longer than eight days, which is considered "clinically significant."

However, all observed people had their cycles return to normalcy by the second cycle after vaccination.

The second observation with 5,688 Norwegians asked if people had any changes such as heavier bleeding or worse pain before and after getting the vaccine. Findings showed 38% of participants had a change before getting vaccinated, which increased to 39% after the first dose and to 41% after the second dose, with the most common change being a heavier flow.

After both observations, Male said the data should be reassuring to those on the fence of getting the vaccine, as changes were short-lived. She also noted that some vaccine hesitancy stems from misinformation like the vaccine is a form of "sterilization."

"If you are particularly worried about your cycle, it’s also worth noting that we do have some evidence that COVID infection can disrupt the cycle, and getting vaccinated could help you avoid this," Male said.

Even with this data, Male said further research is needed on those taking hormonal contraception as it could give an idea if these menstrual changes are driven by sex hormones. Those with pre-existing gynecological conditions may also be more vulnerable to change.

Tomer Singer, medical director at Shady Grove Fertility in New York City who was not involved in the study, says the studies should show benefits outweigh the risk of getting the COVID-19 vaccine. In the past year, he said 5% of the 1,500 patients he's seen in the past year reported menstrual changes, but none had their conception potential significantly altered.

"These two very large observational studies are supporting what we as OBGYN’s and fertility specialists in the community have been observing and discussing with our patients for two years now and supporting other studies that we have conducted with our medical students, residents and reproductive endocrinologist fellows," Singer said. "I would encourage every patient who is in the reproductive age of 18-50 who has concerns regarding the theoretical risks of receiving the vaccine to speak to an OBGYN or seek the opinion of a fertility specialist so they can provide them with reassurance and relevant data."

Follow Jordan Mendoza on Twitter: @jordan_mendoza5.

This article originally appeared on USA TODAY: COVID vaccine and periods: How will the shot affect my menstrual cycle