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How the Covid-19 Vaccine Affects the Menstrual Cycle


The more time passes since the Covid-19 pandemic, the more studies have been introduced regarding the impact of Covid-19 vaccination on our health.

One of the issues concerning many women all over the world is menstruation. Does the vaccination cause any crucial changes? Is there an actual scare? We consulted medical gynaecologist Dr. Susanna Unsworth to analyse the studies and give detailed insight on the topic. 

Following the roll-out of the Covid-19 vaccination, there have been numerous reports of women experiencing changes in their menstrual period: these have included describing heavy bleeds, longer duration of bleeds and longer cycle lengths.

Unfortunately, the formal clinical trials of the vaccination did not collect data relating to menstruation, and the influx of these anecdotal reports over the last year has caused a lot of concern among women, particularly when taken alongside a lot of the misinformation that the Covid-19 vaccination had a negative effect on fertility.

Since the vaccine was introduced, there have been over 36,000 Yellow Card submissions in the UK (this is the system in place for recording adverse incidents associated with medicines and medical devices), reporting changes in periods or unexpected vaginal bleeding occurring after receiving doses of the vaccine.

A similar peak was seen in the US equivalent reporting system. The National Health Institutes dedicated £1.2 million towards research to determine if there was a significant connection between Covid-19 vaccination and menstrual irregularities.

“Much of the initial concern has come from questionnaire-based reports, asking women who experienced menstrual irregularities to come forward and report them. Unfortunately, whilst this data is interesting, the lack of any control group makes it challenging to take anything significant from the findings. It is also clear that menstrual changes are very common generally, regardless of vaccination status,” Dr. Susannah Unsworth explains. 

However, formalized studies have been funded to try and help answer this question. The first main study was published in January 2022 (1). The study looked at women in the US. It monitored six cycles of data extracted from a fertility tracking app, looking at cycles before and after receipt of a Covid-19 vaccination with a control group of unvaccinated women.

The study found a small increase in cycle length, of less than one day, seen in the vaccinated group. The cycle length increase was significantly higher in women who received both doses of the vaccination within one cycle: note there was a shortened interval between vaccinations in the US (3-4 weeks) compared to the European countries (8 weeks). Therefore two vaccinations received in one cycle were possible for women in the US but not Europe.

There has been a global expansion of this study published in September 2022 (2), extended to include women from the UK, Canada and Europe, as well as the US. This study has assessed just under 20,000 women, of which nearly 15,000 were vaccinated, leaving close to 5000 in the unvaccinated control group.

This study further confirmed the data from the previous research, showing a small increase in cycle length of less than one day, seen in the women in the vaccinated group. Again, it showed a longer increase if both doses were received in the same cycle and showed that women with pre-existing longer cycles were more likely to experience the increase. The changes were very similar across all the different vaccine types. Interestingly, neither of these two studies showed any increase in the duration of bleeding itself.

Importantly, both studies showed that the cycle length changes returned to pre-vaccination levels quickly, with most returning to normal within two cycles.

Studies looking at the reports of heavy bleeding have been less convincing. A Norwegian study published in January 2022 (3) seemed to confirm heavier bleeding and longer duration of bleeding in its study population but commented on a high level of menstrual issues within the group before the vaccine was received. Other smaller studies have also shown a limited effect on blood flow following Covid-19 vaccination (4). Interestingly, one study did suggest that those taking hormonal contraception appeared to be protected from any changes in their cycle (5).

“The data available so far appears to be very reassuring in relation to menstrual changes associated with Covid-19 vaccination. Obviously, any change to a woman’s period can cause an element of concern. But the findings from these studies have shown that the changes associated with Covid-19 vaccination appear to be very small and resolve very quickly, with the most effect seen on cycle duration rather than the degree of blood loss,” says dr Susannah Unsworth and adds:

“As a doctor working in community gynaecology, this data is hugely helpful when it comes to counselling women about what to expect after their vaccination. I think women may experience a slight delay in their period after they receive their vaccination, but their cycle length should return to normal within the next two cycles. There is no significant evidence to show that their period would be any heavier than normal. Therefore, if they do experience any significant changes in their cycle length or flow that persists beyond two cycles, they should speak to their doctor about this as it is unlikely connected to their vaccination and may warrant further assessment.”

It has been suggested that fears around changes to menstruation are one of the reasons some women choose not to receive the Covid-19 vaccination. Hopefully, making women aware of the small and temporary changes they might expect will help reduce that fear, especially as it is likely that significant illness following Covid-19 infection is far more likely to impact their menstrual health.

(1) 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 US Cohort. Obstet Gynecol. 2022 Apr 1;139(4):481-489. doi: 10.1097/AOG.0000000000004695. Epub 2022 Jan 5. PMID: 34991109; PMCID: PMC8936155.

(2) Edelman A, Boniface ER, Male V, et al

Association between menstrual cycle length and covid-19 vaccination: global, retrospective cohort study of prospectively collected data

BMJ Medicine 2022;1:e000297. doi: 10.1136/bmjmed-2022-000297

(3) Trogstad, Lill, Increased Occurrence of Menstrual Disturbances in 18- to 30-Year-Old Women after COVID-19 Vaccination (January 1, 2022). Available at SSRN: https://ssrn.com/abstract=3998180 or http://dx.doi.org/10.2139/ssrn.3998180

(4) Namiki T, Komine-Aizawa S, Takada K, Takano C, Trinh QD, Hayakawa S. The association of three doses of the BNT162b2 mRNA vaccine with abnormal bleeding and an irregular menstrual cycle among premenopausal females: A single institute observation study. J Obstet Gynaecol Res. 2022 Nov;48(11):2903-2910. doi: 10.1111/jog.15400. Epub 2022 Aug 17. PMID: 36319205; PMCID: PMC9538126.

(5) Alvergne A, Woon EV, Male V. Effect of COVID-19 vaccination on the timing and flow of menstrual periods in two cohorts. Front Reprod Health. 2022 Jul 25;4:952976. doi: 10.3389/frph.2022.952976. PMID: 36303656; PMCID: PMC9580734.

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