TL:DR: We don’t know, but researchers are actively looking into it!
The clinical trials of the vaccines did not ask this question specifically, but many people have reported changes in menstrual cycles following vaccination on social media. There is no evidence that vaccination impacts fertility or that something dangerous is going on. Menstrual cycles can change for a bunch of different reasons and typically go back to normal all on their own. Talk to your clinician if menstrual cycle changes do not improve over time.
A whole lot of people who menstruate went on social media to talk about changes in their periods after getting one of the COVID-19 vaccines. Some reported heavier periods, periods earlier than normal, bleeding between periods, or missing a period. Whether the vaccine is causing the changes is not yet known, but definitely worth looking at! The original clinical trials didn’t ask this question specifically (which is all too common 😡). Fortunately, Drs. Lee and Clancy are asking about menstrual experiences of people after COVID-19 vaccination. Historically, reproductive and menstrual changes have not been a priority in research (which is messed up since this is an important part of health) and people reporting changes may have felt dismissed and overlooked.
Research into this question is an key way to make sure that everyone is heard and included. We eagerly await their results and thank them for taking this on!
In the meantime, there are some guesses about why this MIGHT happen. One is that the vaccine induces a strong immune response, resulting in inflammation of the lining of the uterus (called the endometrium) and causing changes in menstruation. Another is that the vaccine might lower the number of platelets (the cells in the body responsible for helping blood form clots and stop bleeding) a little, which might result in increased bleeding with menstruation. Still another is the stress of vaccination impacts hormones in the body and immune responses, creating changes in periods. Importantly, whatever is going on, the changes appear to last only a short while before going back to normal.
There is NO evidence that vaccination impacts fertility or the ability to have a baby. There is also no evidence that these period changes have any dangerous impacts on the body.
If you or someone you know is interested in participating in study from Drs. Lee and Clancy, you can check it out here.
Some light period reading 😉:
Lack of Data on Covid-19 Vaccines and Periods Inspired Two Feminist Scientists to Learn More
Can the COVID Vaccine Affect Your Period? Some Women Say It’s Changed Theirs—Here’s What We Know Right Now