We asked a gynaecologist why some people’s periods are being messed up by the vaccine

‘Any effects of the vaccine are likely to be short lived and much less severe than those associated with Covid-19 infection’

Jenny* was ecstatic to finally be offered her jab. She experienced a few minor side effects in the 24 hours after her Covid-19 vaccine, but a week later when she started her period, she noticed another. She told us: “It was so heavy and full of huge clots I would’ve thought I was having a miscarriage were I not a lesbian. Had I known this would’ve happened, I still would’ve got the vaccine but it would’ve been nice to have been given the heads up if it was possible”.

It seems she’s not alone. As well as heavy periods, there have also been reports of women getting their periods later than expected. Grace* had never experienced significant problems with her period but after getting her vaccine, she said: “I literally haven’t had mine since I got vaccinated three weeks ago which makes it over a week late. I was also in agony with mid-cycle bloating and felt super emotional – it’s the first time ever I’ve experienced PMS”.

Meanwhile, Sarah* claimed the vaccine gave her “the lightest period of my entire life – but it did get heavier towards the end which was bizarre”.

So, does the Covid-19 vaccine affect your period? And is there any science to explain what’s going on?

We asked Dr Jacqueline Maybin, a Senior Clinical Research Fellow and Consultant Gynaecologist at the MRC Centre for Reproductive Health at the University of Edinburgh, what’s going on.

Is there any scientific evidence or reasoning as to why people are reporting changes to their periods and menstrual cycles?

“There have been lots of anecdotal reports of people with menstrual disturbance following Covid-19 vaccination, including changes in frequency, duration, regularity and volume of menstruation.

“In the UK, the MHRA have had approximately 4,000 reports of menstrual disturbance via their yellow card reporting system, although actual numbers may be larger due to a lack of awareness of this scheme. Data are also being collected by Dr Kate Clancy at the University of Illinois via a Twitter-based survey.

“At this stage, it is difficult to be certain regarding the mechanisms causing these effects. It may differ from person to person. The brain, ovaries and womb interact to control the menstrual cycle. So, 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 down-regulate to prevent against pregnancy and conserve energy. This brain-level effect may explain some of the changes in menstruation observed during the pandemic or after vaccination.

“The Covid-19 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. The inflammation may also potentially alter how the womb lining breaks down, causing heavier periods. 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, which has also been linked with menstrual disturbance. Women who are called for the vaccine shouldn’t be deterred from attending.”

Why do you think this hasn’t been formally listed as a potential side effect?

“The MHRA have stated that expert review of the data collected so far has concluded that the number of reports of menstrual disturbance are low compared to the number of people vaccinated. This highlights the need for those noticing these menstrual effects to report them using the yellow card reporting system. Data are essential here.

“If there is not an increased risk of menstrual disturbance with the vaccine, then adding this as a potential side effect may result in some women not seeking help for causes of menstrual disturbance that require prompt treatment. Current advice is to speak to your doctor if you are worried about unusual menstrual bleeding. However, if a clear association emerges, then it should be listed to prevent unnecessary worry about short term changes following vaccination. ”

Are those taking hormonal contraception more or less likely to experience changes to their cycles?

“Women using hormonal contraceptives have also reported menstrual disturbance. We don’t yet know if they are more or less likely to experience unusual bleeding than those not taking hormonal preparations. Again, collecting this data is vital, alongside studies to determine the mechanisms involved, which may differ in those taking hormones.

“It is also important to highlight the potential impact for women using natural family planning. If the menstrual cycle is disrupted post-vaccination these methods will become much less effective at preventing pregnancy. The date of ovulation is pretty fixed if you work backwards from the day you get your period but the first half of the cycle varies. Therefore, it is only accurate in retrospect. If predicting safer times on previous cycles, one disrupted cycle could make the method much less reliable. This includes so-called contraceptive apps like Natural Cycles as anything that relies on dates will be skewed by a disrupted menstrual cycle.”

Is there a difference between how different brands of Covid-19 vaccine are affecting people’s period?

“Out of the 4,000 reports of disturbances made to the MHRA up to 17th May 2021, there are 2,734 reports of period problems linked to the AstraZeneca vaccine, 1,158 related to the Pfizer jab, and 66 linked to the Moderna vaccine.”

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