Women who found their menstrual cycle changed after having their Covid-19 vaccinations will be relieved to know thousands of other women also experienced this. A new study of more than 19,000 women has found that Covid-19 jabs are linked to a "small and likely to be temporary change in menstrual cycle length" and women who had two vaccines during the same menstrual cycle were the most affected.
Thousands of women had reported changes to their periods after having a Covid-19 vaccination - but this was not listed as a common side effect of the vaccine. Nevertheless, primary care clinicians and staff working in reproductive health were "increasingly approached" by people who experienced changes in their monthly cycle shortly after a vaccination, as reported by Dr Victoria Male, a lecturer in reproductive immunology at Imperial College London.
In an op-ed published in the British Medical Journal (BMJ) last September, Dr Male said more than 30,000 reports had been made to the UK Medicines and Healthcare Products Regulatory Agency (MHRA) by people who said their menstrual cycle had been affected by their Covid-19 vaccinations. Dr Male said this link was "plausible" and "should be investigated" - but also noted that the majority of people who reported changes to their menstrual cycle later reported that their cycle returned to normal on their next cycle. Importantly, Dr Male also noted that there was no evidence to suggest that Covid-19 vaccinations adversely affected fertility.
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Now, the outcome of a large-scale study into the association between menstrual cycle length and Covid-19 vaccinations has been published. The BMJ has published a research paper by Dr Male and several other experts, led by Dr Alison Edelman of the department of obstetrics and gynaecology at Oregon Health & Science University in the US, which details changes to surveyed women's menstrual cycles post-vaccination.
Using menstrual cycle tracking application Natural Cycles to plot the length of time between their periods pre- and post-vaccination, participants were able to share with researchers how the vaccines impacted their menstrual cycles.
The research paper, which you can read online here, found that individuals who were vaccinated "had a less than one day adjusted increase in the length of their first and second vaccine cycles, compared with individuals who were not vaccinated". The exact figure for the increase to a woman's cycle was given as 0.71 of a day. For the latest Covid stats subscribe to our newsletters here.
But it was also reported that a woman's menstrual cycle was further lengthened if she had two vaccines during the same cycle. "The adjusted difference was larger in people who received two doses in a cycle (3.70 days increase (2.98 to 4.42))," the research paper reports.
The study concluded that "Covid-19 vaccination is associated with a small and likely to be temporary change in menstrual cycle length but no change in menses length".
"Our findings from this large international cohort of individuals continue to be reassuring and can be used to counsel individuals about what to expect with a covid-19 vaccination and how to make an informed decision about vaccination versus continuing to be at risk for covid-19 disease and its related morbidity and mortality," researchers added.
"Although we do find menstrual changes after covid-19 vaccination, these changes are small compared with normal variation and resolve in the cycle after vaccination, except in people who received both doses in one menstrual cycle. Future work should assess other aspects of changes to menstrual cycles, such as unexpected vaginal bleeding, menstrual flow and pain, and define the mechanism by which the post-vaccination menstrual changes described here occur."
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