A study found a temporary relationship between menstrual (period) cycle length and COVID-19 vaccination. Compared to those who were unvaccinated, people who were vaccinated with any of the COVID-19 vaccines had a slightly longer menstrual cycle (less than one day longer), and those who received both doses of the vaccine within one menstrual cycle had a 4 day longer cycle on average (BMJ Medicine).  This change in cycle length disappeared by the next menstrual cycle. The vaccines are still safe for people who are pregnant or trying to get pregnant.

U.S. and international studies linked the COVID-19 vaccine with temporarily longer menstrual cycles, or the number of days between the first day of menstruation (period) to the first day of the next menstruation. After getting the vaccine, study participants got their period one day later than usual; those who received both vaccine doses within one menstrual cycle experienced up to a 4 day delay on average.

However, participants who experienced longer menstrual cycles bled for the same number of days each time and this change in cycle length typically went away before their next period.

Other research has found that the COVID vaccine can cause heavier bleeding or longer-lasting periods, but these changes are also temporary. 

The temporary changes to menstrual cycles seen after COVID vaccination are smaller than temporary changes linked with activities like excessive exercise and health conditions like hormonal imbalances and diabetes.

Getting vaccinated does not affect a person’s ability to get pregnant, according to the best available research. Studies have found no difference in pregnancy success rates between vaccinated and unvaccinated people, including for people using assisted reproductive procedures like in vitro fertilization. Vaccines are also safe for people who are breastfeeding.

The Centers for Disease Control and Prevention continue to recommend that people who are pregnant, breastfeeding, or might get pregnant in the future get vaccinated and boosted against COVID-19. Getting sick with COVID during pregnancy raises a patient’s risk of pregnancy complications, so getting vaccinated is a key tool for keeping both the baby and the parent safe.

Further research is still needed to understand the cause of these menstrual cycle changes. The study referenced here suggests temporary activation of the immune system response may be the reason.

The referenced study only included persons with menstrual cycles not using hormonal contraception; additional research is needed to understand menstrual cycle changes due to the COVID-19 vaccine in persons using hormonal contraception methods.