What We’re Learning About Covid-19 Vaccination and the Menstrual Cycle

If you thought it affected your cycle, you were right. Here’s what’s going on. 

I just got boosted. Aside from a sore arm (which I had to put a brave face on for, so my daughter wouldn’t be afraid to get her shot after me), I didn’t have too many side effects. After my first and second shots I had no side effects other than feeling like someone punched me hard in the arm, which is different from what so many other women are reporting. (I was a geek and timed my vaccine with my menstrual cycle, read on…) 

Women have been reporting menstrual side effects since the shots rolled out. Unfortunately (and unsurprisingly), clinical trials of the current Covid-19 vaccines did not collect menstrual cycle outcomes as part of their research. That means changes in the menstrual cycle were not listed as a known side effect of the vaccines. When women began experiencing disturbances in their menstrual cycle and sharing them online, it led to questions and concerns about whether this was normal and if there were concerns regarding potential impacts on fertility. 

So I’m very glad to see research finally addressing these issues. 

First, let’s state upfront that the Covid-19 vaccines do not impact fertility. A study of more than 2,000 couples funded by the National Institutes of Health and published this month in the American Journal of Epidemiology found no difference in the chances of conception if either female or male partner had been vaccinated compared to unvaccinated couples. 

Interestingly, couples did have a slightly lower chance of conception if the male partner had been infected with Covid within 60 days before his partner’s menstrual cycle, which suggests that Covid-19 could temporarily reduce male fertility. 

Now let’s look at the vaccines and the menstrual cycle. A study of nearly 4,000 women published earlier this month in Obstetrics & Gynecology reported that on average, women’s cycles were delayed by just over half a day following the first dose and nearly one day following the second dose. 

The researchers arrived at those conclusions by analyzing data from 2,403 vaccinated women and 1,556 unvaccinated women who had tracked their menstrual cycles for at least six months. It’s important to note that while this study’s results found a small average change in cycle length, there were women in the study who experienced greater impacts to their cycle. 

Specifically, a subgroup of women who had received both of their vaccine doses within a single menstrual cycle had more pronounced side effects. This group experienced a change in cycle length of at least two days. Some of them had an increase in cycle length of eight days or more. 

Outside of looking at that subgroup who got both shots in one cycle, this study did not take into consideration what phase of the menstrual cycle the women were in when they received their shots, which could be meaningful. I have been researching into this (and have an ongoing study with the help of WILD.ai) and have found that if the shot is done in the low hormone phase of the cycle there is no change. But if it is done after ovulation in the luteal phase, we’re seeing heavier, more prolonged bleeding in their next period, missed menstrual cycle, and/or a change in length by +/- 5 days (i.e. the period comes within days after the shot or is delayed about 5 days).  

It’s not surprising that these shots could lead to some temporary disruption, because for one, they create a stressful event in the body as the immune system mounts a response to the vaccine, and it’s well-known that stress can impact the menstrual cycle. Also, the immune system and the reproductive system communicate with each other in the body, so what affects one can affect the other.   

What does this all mean? For one, if you’ve experienced disruptions to your menstrual cycle following your vaccination, it is not in your head. It also should not deter you from getting vaccinated, because these changes are temporary. The study found that the changes in cycle decreased in subsequent cycles, even among the women who experienced more pronounced changes. 

If you’re scheduling your booster, and you have a chance to do so during the low hormone phase, that’s a good time for the body’s immune system to respond without disrupting your cycle. After ovulation, the body may rely more on pro-inflammatory cytokines for the immune response and you may experience more disruption to your cycle. But either way: getting your shots is the most important thing here. Any menstrual cycle side effects are temporary.