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Published Research

Following up on Covid Vaccines' Effect on Menstrual Cycles

Illustration of a vial of vaccine

Key takeaways

  • On average, there were no significant changes to cycle length for those who received the vaccine over the course of two or more cycles. 
  • Those who received both vaccine doses in the same cycle had an average increase in cycle length of almost four days. Although this is important, it is not clinically significant given that an individual's cycles vary naturally by more than the amount found post-vaccine.

Many people are interested in whether or not Covid vaccines have an effect on the menstrual cycle. Here at Natural Cycles, we conducted a study including around 4000 of our own users in the US, which found that some users experienced a small and temporary change in their cycle length, which soon returned to normal.

Given this first study’s size, and that it only included participants in the US, we decided to follow it up with another, larger, study. This included users from more countries, in order to investigate if the results were applicable to a larger group. 

This second study included 19,622 Natural Cycles users, 14,936 who were vaccinated and 4,686 who were not. We had asked our users to log in the Natural Cycles app when they were given their vaccines, as well as which brand of vaccine they got. 

As with the first study, we compared the cycle length of those who were vaccinated with cycles from those who were not, with the aim to see if there were any changes in cycle length and period length after vaccination. Additionally, we wanted to see if the type of vaccine mattered for the results.

At the end of the study, the results showed that the average cycle length increased by less than one day after vaccination, and this change then went away in the following cycle. For those who got both vaccine doses during the same cycle, the average cycle length increased by almost four days. In the following cycle, the change had almost gone away completely. 

Period length was not affected by vaccination, and there was no difference in results depending on the different vaccine types administered.

In summary, this study found that on a group level, there was a small change in cycle length following vaccination. However, as with the first study, the change was less than one day which means that most users didn’t experience any noticeable change to their cycle.

Why do we do this type of research? Well, it’s valuable to know what to expect after vaccination, and the results of this study can help reassure those who are looking to get vaccinated, plus it can help them make an informed decision together with their healthcare professional. While a small and temporary change to the cycle can happen, menstrual cycles also vary naturally and the change following the vaccines were small in comparison.