Covid vaccines can cause small changes to the menstrual cycle but it quickly returns to normal, a new study has found.
Dr Victoria Male, a lecturer in reproductive immunology at Imperial College London, said research from the US and Norway was reassuring for women who have received a vaccine.
The Medicines and Healthcare products Regulatory Agency (MHRA), received more 37,000 complaints of heavy bleeding and delayed periods after a Covid jab.
However such changes are not thought to affect fertility and Dr Male blamed misinformation surrounding the vaccine for fuelling concerns.
Writing for the British Medical Journal, Dr Male said: “Much of the public concern around this issue arises from misinformation that Covid-19 vaccines cause female infertility.
“Although we already have evidence that this is not the case, it comes from the clinical trials, in which pregnancy rates were extremely low because participants were using contraception, and fertility clinics, where users do not necessarily reflect the broader population.”
Dr Male added: “Changes to the menstrual cycle do occur following vaccination - but they are small compared with natural variation and quickly reverse.”
The MHRA has insisted there is no link to between the vaccine and infertility.
In the US study, 3,959 Americans logged at least six consecutive menstrual cycles, of which 2,403 were vaccinated.
The first dose of the vaccine had no effect on the timing of the subsequent period, while a second dose delayed the cycle by half a day.
There was a two-delay for those who had two vaccine doses within the same cycle but Dr Male said this would be unlikely for women in the UK, where the gap between vaccine doses is at least eight weeks.
One in ten women did report a change in cycle by more than eight days but after two cycles, it returned to normal.
A separate study in Norway showed how menstrual cycles can vary, with at least 40% reporting a change before being vaccinated.
Results of a study involving British women using the menstrual-cycle tracking app are also expected to be revealed shortly.