Disruption to the menstrual cycle after Covid vaccines is likely to be short-lived but studies are ongoing, a Scottish reproductive expert has said.

Up to 20-30% of women reported some changes to periods following vaccines in one study, including a change in cycle length and regularity.

Another, US study published this week, found that people who receive both doses of an mRNA vaccine, such as Pfizer, in the same cycle experienced, on average, a delay to their next period of two days.

The International Federation of Gynaecology and Obstetrics classifies a variation in cycle length as normal if less than eight days.

HeraldScotland:

Data was analysed for 2,403 vaccinated individuals and 1,556 who were unvaccinated and in those who experienced some disturbance, periods returned to normal within two cycles. 

People receiving their first dose of a vaccine did not experience a delay to their periods, and those receiving their second dose experience on average less than half a day’s delay.

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Dr Jackie Maybin, a consultant gynaecologist and research fellow at the University of Edinburgh, said scientists are not yet sure of the mechanisms which are causing the disturbances and studies are ongoing.

One possible theory is that mRNA vaccines create a robust immune response or stressor, which could have a temporary effect on the hormones that regulates menstrual timing.

HeraldScotland:

"The brain, ovaries and womb interact to control the menstrual cycle," said Dr Maybin.

"Menstrual disturbance may be due to effects on the part of the brain that controls the reproductive hormones, effects on the ovaries or effects directly on the lining of the womb (which is what is shed during a period). 

"In times of stress, the female system is designed to temporarily downregulate to prevent against pregnancy and conserve energy. 

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"This brain-level effect may explain some of the changes in menstruation observed during the pandemic, with Covid-19 or with vaccination.

"The vaccination itself is designed to initiate an immune response in the body to protect against future Covid-19 infection. 

"The resulting inflammation may transiently affect the ovaries, altering their hormone production over one or two cycles, resulting in irregular or heavier menstrual bleeding. 

HeraldScotland:

"The inflammation may also temporarily alter how the womb lining breaks down and sheds, causing a heavier period. 

"These effects could lead to temporary changes in menstrual symptoms that should spontaneously resolve. It is important to emphasise that any effects of the vaccine are likely to be short lived and much less severe than those associated with Covid-19 infection."

She said similar, short-term menstrual effects had been reported following other vaccinations including HPV and Hepatitis B.

"This suggests that effects are due to either the stress of receiving a vaccination or due to the necessary immune process that occurs to initiate protection," she said.

"These previous reports are of transient effects on menstrual symptoms and there is no evidence of an impact on fertility. 

"When we combine these findings with the data emerging on Covid-19 vaccination and menstrual cycles, any effects on periods are likely to be short lived and outweighed by the benefit of protection against severe Covid.

"Women who are called for the vaccine shouldn't be deterred from attending."