A study published Thursday found that women’s
menstrual cycles did indeed change after vaccination against the coronavirus.
The authors reported that women who were inoculated had slightly longer
menstrual cycles after receiving the vaccine than those who were not vaccinated.
Their periods, which came almost a day later
on average, were not prolonged, however, and the effect was transient, with
cycle lengths bouncing back to normal within one or two months. For example,
someone with a 28-day menstrual cycle that starts with seven days of bleeding
would still begin with a seven-day period, but the cycle would last 29 days.
The cycle ends when the next period starts and would revert to 28 days within a
month or two.
The delay was more pronounced in women who
received both vaccine doses during the same menstrual cycle. These women had
their periods two days later than usual, researchers found.
The study, in the journal Obstetrics &
Gynecology, is one of the first to support anecdotal reports from women that
their menstrual cycles were off after vaccination, said Dr Hugh Taylor, chair
of the department of obstetrics, gynecology and reproductive sciences at Yale
School of Medicine.
“It validates that there is something real
here,” said Taylor, who has heard about irregular cycles from his own patients.
At the same time, he added, the changes seen
in the study were not significant and appeared to be transient.
“I want to make sure we dissuade people from
those untrue myths out there about fertility effects,” Taylor said. “A cycle or
two where periods are thrown off may be annoying, but it’s not going to be
harmful in a medical way.”
He had a different message for postmenopausal
women who experience vaginal bleeding or spotting, whether after vaccination or
not, warning that they may have a serious medical condition and should be
evaluated by a physician.
One serious drawback of the study, which
focused on US residents, is that the sample is not nationally representative
and cannot be generalised to the population at large.
The data was provided by a company called
Natural Cycles that makes an app to track fertility. Its users are more likely
to be white and college educated than the US population overall; they are also
thinner than the average American woman — weight can affect menstruation — and
do not use hormonal contraception.
For women in their childbearing years, the
findings should be reassuring, said Dr Diana Bianchi, the director of the
Eunice Kennedy Shriver National Institute of Child Health and Human
Development. (The National Institutes of Health’s Office of Research on Women’s
Health and NICHD helped fund the study, as well as related research projects at
Boston University, Harvard Medical School, Johns Hopkins and Michigan State
“Their providers can say, ‘If you have an
extra day, that is normal. It’s not something to be concerned about,’” Bianchi
The study was carried out by researchers at
Oregon Health & Science University and the Warren Alpert Medical School of
Brown University, in collaboration with investigators from Natural Cycles,
whose app is used by millions of women around the world.
De-identified data from users who consented to
have their information incorporated into the research provided a trove of
evidence about how women’s cycles changed during the pandemic.
Researchers looked at records from nearly
4,000 women who had meticulously tracked their menstruation in real time,
including about 2,400 who were vaccinated against the coronavirus and about
1,550 who were not. All were US residents ages 18 to 45 who had logged their
periods for at least six months.
For those who were vaccinated, researchers
examined the three cycles before and after the vaccine to look for changes,
comparing them with a similar six-month duration in women who did not receive a
Overall, vaccination was associated with less
than a full day’s change in cycle length, on average, after both vaccine doses,
compared with pre-vaccine cycles. The unvaccinated group saw no significant
changes over the six months.
Future studies using the database will examine
other aspects of menstruation, such as whether periods were heavier or more
painful after vaccination.
The findings of the new study may not apply
equally to all women. Indeed, much of the change in cycle length was driven by
a small group of 380 vaccinated women who experienced a change of at least two
days in their cycle, said Dr Alison Edelman, a professor of obstetrics and
gynecology at Oregon Health & Science University and the paper’s lead
Some women who were vaccinated had cycles that
were eight days longer than usual, which is considered clinically significant,
“Though the cycle length was less than one day
different at the population level, for an individual, depending on their
perspective and what they’re relying on menses for, that could be a big deal,”
she said. “You might be expecting a pregnancy, you might be worrying about a
pregnancy, you might be wearing white pants.”
It’s not clear why the menstrual cycle might
be affected by vaccination, but most women with regular periods experience an
occasional unusual cycle or missed period. Hormones secreted by the
hypothalamus, the pituitary gland and the ovaries regulate the monthly cycle,
and they can be affected by environmental factors, stressors and life changes.
(The changes observed in the study were not
caused by pandemic-related conditions, the authors said, since women in the
unvaccinated group were also living in the pandemic.)
Whether other vaccines affect menstruation is
not known — clinical trials of vaccines and therapeutics do not generally track
menstrual data points, unless investigators are specifically testing
therapeutics as contraceptives or fertility enhancers, or they want to rule out
“We’re hoping this experience will encourage
vaccine manufacturers and clinical trials of therapeutics to ask questions
about the menstrual cycle, the same way you’d include other vital signs,”
The information is important, just like
knowing that one may experience a headache or develop a fever after
vaccination, Edelman said.
“Individuals who menstruate spend a week out
of every month, sometimes more, having to deal with menstruation,” Edelman
said. “If you add up the time over 40 years, it’s practically 10 years of
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