Since covid vaccination has been promoted worldwide (with billions of women vaccinated to date), anecdotal reports have spread, especially via social media suggesting a potential link to changes in menstrual patterns amongst vaccinated women.
It’s a difficult topic to assess as many sources of bias can affect our observations. For example, even in women with a “clockwork” menstrual cycle, variations in cycle length month to month up to 8 days are considered within the normal spectrum.
In addition to this, menstrual cycle irregularities and abnormalities are fairly common due to issues such as PCOS, fibroids, stress, hormonal concerns, weight modulation, insulin resistance, bleeding concerns due to fibroids, polyps, cervical concerns, biochemical pregnancies and early miscarriages – to mention a few. This means that menstrual abnormalities develop commonly, regardless of vaccination. During a period of time where population mass vaccination campaigns are underway, some of these unrelated causes may be falsely attributed to the effects of vaccination.
A recent study was conducted to try to clarify observations and concerns, using data from the Natural Cycles App. Study participants were women who had been actively tracking their menstrual cycles before and after Covid vaccination.
3959 women in the study were aged 18 to 45 years, were not using hormonal contraception and had cycle length 24-38 days (normal range defined by the study). Six consecutive cycles were considered. The study ran between December 2020 and July 2021.
In Australia, a 38 day cycle would not be considered normal length, so a study weakness is that some women in this study by our criteria would already be considered to have irregular cycles.
2403 vaccinated women were the study group (55% Pfizer-BioNTech vaccine, 35% Moderna, 7% Johnson/Janssen) – with 3 cycles tracked before vaccination and 3 after.
1556 unvaccinated women were the control group – with six cycles monitored consecutively.
A medically and statistically significant change in menstrual cycle length was defined as 8 days or more (variation within this margin cycle to cycle is considered within the normal spectrum).
What did the study find?
For the first dose of vaccination – on average no statistically significant change in menstrual cycle length was recorded.
The proportion of women who experienced a significant change in cycle length was 4.3% for unvaccinated and 5.2% for vaccinated women. This finding was not statistically significant.
For the second dose of vaccination, again on average no statistically significant change in menstrual cycle length was recorded.
The proportion of women who experienced a significant change in cycle length was 4.6% for unvaccinated and 6.5% for vaccinated women.
Interestingly, for a small subset of women who received two doses of vaccine in the same menstrual cycle, a higher proportion (10.6%) experienced significant menstrual cycle delay ( > 8 days increase in cycle length). This quickly normalized in the following cycles. Conversely this effect was not seen in 89.4% of women in this subset. Potential confounding factors include stress effects of lockdowns and weight changes which can also affect menstrual cycle regularity. Still, this effect although self limiting could be a real vaccine side effect for those who received 2 doses in the same month.
What is the take home message?
Covid vaccination is considered safe and recommended for all women considering pregnancy and who are pregnant.
The overwhelming majority of covid vaccinated women studied experienced no significant change in menstrual cyclicity around the time of vaccination.
It is possible although not proven that a small minority of women who receive two vaccine doses in a single month (not recommended/offered in Australia) may experience a very temporary menstrual cycle change. This was not seen in women receiving two doses across two separate menstrual cycles which is what we are offered in Australia.
My advice, RANZCOG and ANZSREI’s advice, ATAGI’s advice remains – get vaccinated, get boosted and stay safe.
Reference: Edelman A, Boniface ER, Benhar E, Han L, Matteson KA, Favaro C, Pearson JT, Darney BG. Association Between Menstrual Cycle Length and Coronavirus Disease 2019 (COVID-19) Vaccination: A U.S. Cohort. Obstet Gynecol. 2022 Jan 5. doi: 10.1097/AOG.0000000000004695. Epub ahead of print. PMID: 34991109.