top of page

Can the COVID vaccine affect your menstrual cycle or fertility? Here's what the science says

Many a conversation has been happening in clinic surrounding this question. Many patients have been concerned about a possible impact on their fertility from the Covid vaccine. A few patients, including myself have indeed seen some alteration to our menstrual cycles following the Pfizer vaccine (not sure about Astra Zeneca though). My cycle, following the 2nd Pfizer, came 4 days later than usual and was possibly a little bit heavier. It has however, come back to my regular 28 days for the next cycle. I am always curious to find out the science and physiology behind the female reproductive system, so I started to research.

I read an article written by the ABC last week that may give us some clarity and possibly peace of mind.

As COVID-19 vaccines continue to roll out around the world, a small but growing number of women, including me, have reported short-term changes in their menstrual cycle following vaccination. But drawing a connection between menstrual patterns and vaccination is far from straightforward.

Despite anecdotal reports, there is no scientific evidence that links menstrual irregularities to the coronavirus vaccines, and there are many factors that can affect menstruation, including stress.

"We're already on a baseline of people's cycles varying normally, and in the context of the pandemic, people's cycles have been varying more … and that almost certainly has to do with stress," said Victoria Male, a reproductive immunologist at Imperial College London.

Dr Male said there may be "biologically plausible ways" in which vaccines could temporarily impact menstrual cycles.

But other experts have cautioned drawing a link between the two, and say there are much more likely explanations for the uptick in anecdotal reports, which have little to do with the vaccines themselves.

So what might be going on here?

Let's first take a look at the menstrual changes people have said they've noticed, most of which appear to be short-lived.

"The most common thing people report is a heavier period than usual, and the next most common thing is a later period," said Dr Male, who is running a study in the UK investigating whether these short-term changes could be linked to vaccination. "Most people say that happens for one cycle … some people say they have two periods out of whack."

Because COVID-19 vaccine clinical trials didn't track data on menstruation, researchers have been relying on vaccine safety-monitoring systems and self-reporting. In the UK to date, a total of 27,510 reports of "a variety of menstrual disorders" have been reported, "including heavier than usual periods, delayed periods and unexpected vaginal bleeding" following COVID-19 vaccination. That might sound like a lot — but it's on the background of 43.4 million COVID-19 vaccine doses being administered to women.

According to the UK's national drug regulator, the number of reports is, "low in relation to both the number of females who have received COVID-19 vaccines to date, and how common menstrual disorders generally are".

Leading reproductive immunologist Sarah Robertson said this wasn't to say reports of menstrual changes post-vaccination should be dismissed or minimised. But she said it was important to separate instances of correlation (where two events coincide) from causation (where one thing causes another). "There will be lots of people who link [vaccination and menstrual changes] together in their mind," said Professor Robertson of the University of Adelaide. "We are all instinctively, constantly reviewing what's happening with our reproductive cycles, and so when there are unusual events or exposures, we sort of link that with the changes we see in our biology.

Patterns of menstruation can be influenced by a range of factors, including age, medication, illness, diet, exercise habits — and stress. Professor Robertson said if menstrual changes were found to be occurring at higher rates than normal following COVID-19 vaccination, it was most likely the result of "pandemic-induced stress".

"COVID-19, and the stress that comes with [lockdowns] and the disruption to our normal social, family and working lives, can be quite severe," she said.

High rates of stress have been shown to suppress hormone levels in the brain that help to regulate ovarian function, and cause disruptions to the menstrual cycle through this pathway. Professor Robertson said for some people, vaccination itself may be a particularly stressful or anxiety-provoking experience. "There's an extra fear, unfortunately because of the way [Australia] has handled the rollout of the vaccination … that means at the particular day or week [someone] is having the vaccine, they're probably just that little bit more anxious," Professor Robertson said.

But she said there were also "biologically plausible ways" in which the vaccines may be having a more direct impact (albeit temporary) that meant the link was "worth investigating". "We know, for example, the immune response affects sex hormones, and sex hormones affect the immune response," she said. "So we can imagine a situation where if you really sort of activate the immune response, then you might see some short-term changes to sex hormones that would have a knock-on effect on the menstrual cycle."

She said it was also possible a significant activation of the immune system could impact the immune cells in the lining of the uterus, which may have an effect on the heaviness and timing of bleeding. But Professor Robertson disagreed, and said it was highly unlikely that the degree to which the immune system was activated by a vaccine would be enough to alter immune cells in the ovary or uterus (enough to change when or how you bleed), or to change sex hormone levels in "a meaningful way".

"The changes in the immune system in normal circumstances are too small, and too far away from these distant sites to cause any effects that would result in menstrual changes," she said.

"It is not beyond the realms of biological possibility that a severe response to a vaccination could cause small fluctuations to immune cells in the uterus or ovary … or brain tissue that controls the reproductive system. "However, I think this is highly unlikely in all but the most rare cases.

"It's much more likely that other factors explain the menstrual changes that women report."

Research shows no impact on fertility

Both experts hastened to add that it was important not to confuse temporary menstrual changes with long-term fertility. There is currently no evidence that COVID-19 vaccines cause any adverse effects on reproductive or pregnancy outcomes, and vaccination may even reduce the incidence of stillbirth. A US study of over 35,000 pregnant women who received an mRNA-based vaccine (such as the Pfizer jab) found pregnant and non-pregnant women experienced similar side effects.

The chance of serious events like miscarriage and placental abnormalities occurred at a similar rate across both groups.

In Australia, pregnant women are recommended to get vaccinated against COVID-19 at any stage of pregnancy, and women who are breastfeeding or planning to get pregnant are also encouraged not to delay their vaccine.

Professor Robertson said large studies had consistently shown vaccines were safe, highly effective, and had no impact on fertility.

So that's good news right? If you have any concerns or queries, please feel free to call or email us and we can talk through your concerns and support you through this very stressful time in our lives.

Petra xx

bottom of page