Live: 300+ Best Black Friday Deals Live: Black Friday TV Deals BF Deals Under $25 BF Deals Under $50 5 BF Splurges 8 BF Must-Haves 15 Weird Amazon BF Deals BF Cheat Sheet
Want CNET to notify you of price drops and the latest stories?
No, thank you

New Study Backs What We Know About COVID Vaccines and Periods

People's periods may be slightly delayed after vaccination. But experts say it's temporary and even makes sense given the number of outside influences on the menstrual cycle.

A hand in a turquoise glove holding a needle
Francesco Carta/Getty Images
For the most up-to-date news and information about the coronavirus pandemic, visit the WHO and CDC websites.

Billions (with a "b") of COVID-19 vaccine doses have been given to people since the pandemic began more than two years ago. But like many research initiatives in science and medicine, the menstrual cycle was largely ignored in early studies on the vaccines, despite it being a vital signal and pattern that often reflects the health (physical and emotional) of people who have one.

But research has started to roll in in recent months, hopefully inspiring more in future medical advancements. A large study published this week in BMJ Medicine landed on similar results from earlier research: there might be a slight, temporary delay in the onset of someone's menstrual cycle (when they get their period) around the cycle they got vaccinated against COVID-19. The average period delay was about one day, and most people's cycles resumed normally again the next month. 

While this is validating for people who did notice a change in their period post-vaccine, that small of a (temporary) change isn't necessarily a big deal in the world of gynecology and reproductive medicine. That's because the hormones that control and orchestrate the delicate dance that is the menstrual cycle can shift based on a variety of factors, including stress levels, whether we're eating enough food, weight gain or loss and, apparently, things that impact our immune systems, like vaccinations and illnesses. 

Here's what we know about COVID-19 vaccination, periods and fertility.

COVID vaccines and periods 

Once COVID-19 vaccines started rolling out to the masses (the biggest vaccination campaign of most people's lifetimes), some people noted that their periods were a little different the month they got their shots. Then a January study published in the Obstetrics and Gynecology Journal confirmed what many people already knew: Getting vaccinated might temporarily delay your period. According to the study, someone with a regular cycle with an average length (24 to 38 days) was delayed by an average of roughly one day. Most people's periods returned to normal in one to two months. 

A large study published this week in BMJ Medicine found a similar trend in people with regular periods who weren't taking hormonal birth control (hormonal birth control interferes with ovulation and impacts whether/when someone experiences a bleed each month). The onset of menstruation was delayed by a little less than one day for the vaccine cycle, but this change was temporary. People who got two vaccine doses within the same menstrual cycle had a longer delay, on average, of about four days.

But will that make you infertile? No. Like other factors that can make a period show up a little differently than normal (like stress, a stretch of heavy exercise or COVID-19 itself) the changes after vaccination seem to be temporary. The changes might also be "small compared with normal variation" of the menstrual cycle, researchers noted. Even in people with regular cycles, a period may not be officially considered late until it's a few days past when you expected it.

But it's important to note, as researchers did in the BMJ study, that the study was done on people who have regular menstrual cycles to begin with, without the use of birth control. People who already have irregular periods or have a medical condition like PCOS that disrupts menstruation may have a different experience. This highlights the need for further research to include menstrual cycle effects in all studies. 

But in real time, you likely don't need to worry if your period is slightly different the month you got a vaccine (unless you think there's another cause for it). "I want to make sure we dissuade people from those untrue myths out there about fertility effects," Dr. Hugh Taylor, the chair of the department of obstetrics, gynecology and reproductive sciences at Yale School of Medicine told the New York Times in January. "A cycle or two where periods are thrown off may be annoying, but it's not going to be harmful in a medical way."

When our bodies experience temporary stress, illness or an abrupt lifestyle change, hormones may be disrupted temporarily. This could be read as nature's way of saying, "right now is not the best time to become pregnant."

Do COVID vaccines make it harder to get pregnant? 

No, according to a February statement from the American College of Obstetricians and Gynecologists, the American Society for Reproductive Medicine and the Society for Maternal-Fetal Medicine. COVID-19 vaccines are encouraged for people who are already pregnant and those who might want to be in the future. 

The US Centers for Disease Control and Prevention says there's currently "no evidence" that any vaccine, including ones for COVID-19, causes fertility problems for either men or women. 

Here's research to back up the agencies' claim: One study published in the Lancet looked at people who got pregnant while participating in trials for AstraZeneca's vaccine (a similar vaccine to Johnson & Johnson's). The researchers found no differences in pregnancy rates or fertility between those who received the COVID-19 vaccine versus the placebo, and also no difference in miscarriage rates. 

Another study published in BioMed Central looked at 36 couples actively trying to conceive using in vitro fertilization. While noting (like many studies on COVID-19) that larger studies should be done to confirm, researchers wrote: "mRNA SARS-CoV-2 vaccine did not affect patients' performance or ovarian reserve in their immediate subsequent IVF cycle." 

But having data that confirms both accidental and planned pregnancies continue to occur in equal numbers, regardless of vaccination status, might ease the initial hesitation around the "new" mRNA technology used in Pfizer and Moderna's COVID vaccines.

"mRNA, specifically, is what people seem to be worried about," Dr. Danielle Jones, an OB-GYN who makes videos debunking COVID-19 myths, said in one video. "mRNA is degraded within your body within hours to days and certainly is gone within a couple of weeks, and all that remains is your body's response, meaning antibodies, to protect you against COVID-19."

Now playing: Watch this: COVID didn't cause a total urban exodus, but it did shift...

Does getting sick with COVID make it harder to get pregnant? 

It depends, current research says. One study in the American Journal of Epidemiology, which looked at heterosexual couples trying to get "spontaneously" pregnant, found that a woman getting sick with COVID-19 didn't impact a couple's chance of conceiving, but a male partner's recent bout with COVID-19 did temporarily lower a couple's chances. 

Although researchers believe it's short-term, couples who were trying to conceive had a slightly lower chance (about 18% less likely) of getting pregnant if the male partner had COVID-19 within 60 days of the reported menstrual cycle period. After 60 days had passed since the partner's COVID-19 infection, there weren't any fertility differences. Women who were tracking their menstrual cycles reported their partner's medical history and vaccination status in the study, though some male partners completed a questionnaire. 

A smaller semen analysis study has also linked COVID-19 illness with poor sperm quality and similar issues, but researchers from that study noted that participants' semen quality prior to getting sick with COVID-19 was unknown. 

The CDC says that fever from illness can lead to a short-term decrease in sperm in otherwise healthy men. "Other possible reasons for a decline in fertility among male partners who recently tested positive could be inflammation in the testes and nearby tissues and erectile dysfunction, all common after SARS-CoV-2 infection," the National Institutes of Health reported in January on the Journal of Epidemiology study. "The researchers noted that this short-term decline in male fertility could potentially be avoided by vaccination."

But what about sperm and the COVID-19 vaccines? One small semen analysis study didn't find a significant difference in the sperm quality of participants before and after two doses of Pfizer or Moderna's vaccines. Participants actually had an increased sperm parameter (more sperm) after vaccination, but researchers attributed it to other factors of the study, not the vaccine. The larger cohort study also didn't find a difference in fertility between unvaccinated and vaccinated men.

New variants of the virus that causes COVID-19 keep emerging, however, which can change the health effects of getting sick with COVID-19. Most people also have some immunity to COVID-19 now, whether it's from prior infection, vaccination or both, which can also influence how a COVID-19 illness impacts you.

Now playing: Watch this: COVID didn't cause a total urban exodus, but it did shift...

The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.