U.S. study offers reassurance on COVID-19 shots, menstrual cycles

One of the first studies to track whether COVID-19 vaccination might affect menstrual cycles found a small and temporary change.

Research published Wednesday tracked nearly 4,000 U.S. women through six menstrual cycles and on average, the next period after a shot started about a day later than usual. But there was no change in the number of days of menstrual bleeding after COVID-19 vaccination.

"This is incredibly reassuring," said Dr. Alison Edelman of Oregon Health & Science University, who led the research and said it's important to tell women what to expect.

Some women have reported irregular periods or other menstrual changes after their shots. The National Institutes of Health is funding studies to examine if there's any link.

Edelman's team analyzed data from a birth control app called Natural Cycles, cleared by the U.S. Food and Drug Administration for women to track their menstrual cycles and tell when they're most likely to become pregnant.

Menstrual cycles are counted from the first day of one period to the first day of the next. Slight variations from month to month are normal — stress, diet and even exercise can spur temporary changes.

Edelman said the study included women with "the most normal of normal" cycle length, averaging between 24 and 38 days. Researchers tracked vaccinated women for three cycles before the shots and the immediate three cycles after, including the months they received a dose — and compared them to unvaccinated women. The app prompted women to enter vaccine information.

A subset of 358 women who got both vaccine doses in the same menstrual cycle saw a slightly larger change to their next cycle length, on average two days. About 10 per cent of them had a change of eight days or more, but subsequently returned to normal ranges, the researchers reported in the journal Obstetrics & Gynecology.

Edelman said one theory is that when the immune system revs up at certain times in the cycle, "our body clock or what controls the menstrual cycle can have a hiccup."

Dr. Deborah Money, a professor of obstetrics and gynecology at the University of British Columbia, agreed the findings were reassuring.

"Menstrual cycle length — even though, you know, some women will swear it's exactly 28 days, or 29 — it varies all the time," she said in an interview.

Stress and other factors can also affect the length of a cycle, Money said, so a one-day difference is not a big deal.

"It's absolutely reassuring, and would suggest that there would be no impact on reproductive health or fertility related to that sort of minor shift."

Money did caution that the people who use a birth control app may not be representative of the entire population.