Covid-19 Vaccines May Affect Your Menstrual Cycle In This Small, Temporary Way

After months of cycling through anecdotal claims that Covid-19 vaccines may be affecting menstrual cycles and calls for more studies, you may be yearning for some real scientific evidence. Well, there’s finally a real scientific study that’s been published in a real medical journal, Obstetrics & Gynecology. And yes the study did find that Covid-19 vaccination could result in some changes to your menstrual cycle. However, before you put a period at the end of that sentence and jump to conclusions, note that the changes were relatively small and temporary.

The study used a female fertility app called “Natural Cycles” to track the menstrual cycles of 3,959 study participants for six consecutive cycles. Each of the participants fell into one of two groups for comparison, either a vaccinated group or an unvaccinated group. For the 2,403 participants in the vaccinated group, the six consecutive cycles consisted of the three consecutive cycles right before their getting Covid-19 vaccine doses and then the three consecutive cycles immediately after vaccination. Of these folks who got vaccinated, 55% ended up receiving the Pfizer-BioNTech vaccine, 35% the Moderna one, and 7% the Johnson & Johnson/Janssen.

For the remaining 1,556 in the unvaccinated group, the study simply followed them through six consecutive cycles. A team from the Oregon Health & Science University in Portland, Oregon, (Alison Edelman, MD, MPH, Emily R. Boniface, MPH; Eleonora Benhar, PhD; Leo Han, MD, MPH; Kristen A. Matteson, MD, MPH, Carlotta Favaro, PhD; Jack T. Pearson, PhD; Blair G. Darney, PhD, MPH) conducted the study.

The two groups were fairly similar in demographics. About 20% of the vaccinated group versus 13% of the unvaccinated group lived in the Northeast, and 37% of the vaccinated group versus 34% of the unvaccinated lived in the Western portion of the U.S. Around 54% of vaccinated folks identified as White, compared to about 47% of the unvaccinated. The participants’ ages ranged from 18 to 45 years. Those who ended up getting vaccinated tended to be slightly older with 34% being in the 30–34 year age range compared to 24% of those who didn’t get vaccinated. Greater percentages of those who got vaccinated (79% vs 69%) hadn’t had babies previously and were college educated (77% vs 60%). Of note, all had histories of fairly regular cycle lengths.

So what did the study find? Those who got their first dose of the Covid-19 vaccine had to then, wait for it, wait for it, wait on average 0.71 days longer for their next period than did prior to vaccination. In other words, their menstrual cycle length, meaning the time between bleeding episodes, tended to increase a bit with most increases falling within the 0.47 to 0.94 day range. Those who got their second dose had on average a slightly greater (0.91 day) increase with most increases somewhere from 0.63 to 1.19 days.

Of course, your menstrual cycle ain’t like an atomic clock. It’s not even like a game show clock. It is not always going to be the exact same length all the time and instead, may vary month-to-month to different degrees. Therefore, not surprisingly those who went unvaccinated in the study experienced some degree of fluctuations as well. On average, the unvaccinated group experienced a 0.07 day increase in cycle length between the first three measured weeks and the second three measured weeks. Most changes ranged from a decrease of 0.22 days to an increase of 0.35 days.

Comparing the vaccinated with the unvaccinated in the study still revealed vaccination to be associated with some lengthening of the menstrual cycle. Those who got vaccinated has on average a 0.64 day increase in cycle length after the first dose, with most increases ranging from 0.27 to 1.01 days and a 0.79 day increase after the second dose with most increases ranging from 0.40 to 1.18.

Such changes were still well within the range of natural variation. Doctors usually don’t consider changes of less than a week to be clinically significant. The vast majority of the vaccinated did not experience such clinically significant changes in the cycle length, although 5.2% of the vaccinated group and 4.3% of the unvaccinated group did have cycle length changes of eight days or greater. However, the difference between the vaccinated and the unvaccinated was not statistically significant.

Even when such cycle lengthening was seen, such increases appeared to be only temporary. Among the vaccinated, most of the observed increases occurred in the cycle immediately following vaccination (i.e., the fourth cycle). By the sixth menstrual cycle, only two cycles later, cycle lengths for the vaccinated group for the most part reverted back to where they had been before vaccination.

Again all of the above changes were to the menstrual cycle length, the time between bleeding episodes. The researchers didn’t find any consistent changes after vaccination in menses length, that is the number of days that study participants experienced bleeding.

So what does this all mean? Well, it wasn’t super surprising to see some changes in menstrual cycle length in the cycle immediately following vaccination. After all, your menstrual cycle depends on chatter among different parts of your body such as your hypothalamus, pituitary glands, ovaries, and the lining of your uterus. Instead of text messages, emojis, and TikTok videos, these body parts use hormones to communicate with each other such as estrogen, progesterone, gonadotropin releasing hormone (GnRH), follicle stimulating hormone (FSH), and luteinizing hormone (LH). Changing levels of these different hormones are what leads to the different steps in your menstrual. And lots of different physical, emotional, and environmental stresses around you can affect these hormone levels.

Speaking of stress, think of vaccination as a bad date for your immune system. The vaccines basically introduce spike proteins to your immune system by providing tiny little blueprints to the cells in your body so that your cells can produce such proteins. These are the spike proteins that stud the surface of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), making it look like the end of a BDSM mace, not that you would know anything about such a thing. Once your immune system meets these spike proteins, it then effectively says, “oh, no you didn’t,” and mounts a response against the spike protein. This is analogous to what happens when your Tinder date shows up with a lint roller and an anvil to your first meeting in a restaurant. Your immune system then is better prepared to deal with any further encounters with the spike protein just like you’ll be better prepared the next time a Tinder profile says, “really like lint rollers and anvils.”

Such an experience can bring some momentary stress. This in turn could potentially affect your hormone levels and potentially the lining of your uterus for a short period of time. When it comes changes in your body, duration is important. Short temporary changes happen all the time. It’s the longer term persistent changes that may matter more. Getting one, two, or even three doses of a Covid-19 vaccine is not the same as ingesting a medication or supplement every single day. The contents of a vaccines leav your body fairly quickly.

Does this study once and for all rule out the possibility that Covid-19 vaccines can have any further effects on your menstrual cycle? No, a single scientific study alone is never enough to prove anything. And this study had its limitations. Although 3,959 may seem like a large number when it comes to people in front of you in the restroom line, it’s a reasonable but not an enormous sample size. The study participants weren’t as diverse as the general population and didn’t include those with histories of more variable menstrual cycles. The researchers didn’t examine other aspects of menstruation and cycles such as associated symptoms and bleeding characteristics before and after vaccination. More studies are needed to confirm the findings from the study and provide more insight into any possible relationships between vaccination and your menstrual cycle.

Nonetheless, results from this study did provide even more reassurance that the Covid-19 vaccines won’t substantially affect your menstrual cycle or fertility. There’s no indication that such small temporary changes in your menstrual cycle should affect your fertility in any way. When it comes to getting vaccinated against Covid-19, all of this further suggests that there’s no real danger going with the flow.