Covid vaccines and pregnancy, fertility and periods—sorting myths from proven side effects

We've asked the experts for the truth about Covid vaccines and pregnancy, plus any potential risks for fertitlity

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Covid vaccines and fertility has become a contentious topic, with information—and plenty of misinformation—on the link between the two flooding news sites, forums, and social media feeds.

Last December, just a few days before the world’s first approved Covid-19 vaccine was administered, a blog appeared on a little-known website called Health and Money News. 

The headline read ‘Head of Pfizer Research: Covid Vaccine is Female Sterilization’. In the post, a retired British scientist called Michael Yeadon—who happened to have previously been VP at the pharma giant before leaving ten years ago—apparently declared that the vaccine could cause infertility in women. 

The claim was immediately rubbished by the likes of AFP Fact Check, the Associated Press, Reuters, and independent UK fact-checking organization, Full Fact. It was also disputed by scientists around the world. 

If you look for the original article now, you won’t find it online. The website has been suspended and any concerns over the reported link between the vaccine and female fertility seem not to have made a dent in the global vaccination program. That might have been the end of the story were it not for social media, where fake news spreads fast. 

Andriy Onufriyenko

(Image credit: Andriy Onufriyenko/Getty )

Before the post and the website hosting it were pulled down, people had screenshotted the headline and liked and shared it thousands of times on Facebook, Instagram, and Twitter. Understandably people were beginning to worry. 

As well as fearing a potential impact on female fertility, some people have questioned whether the vaccine is safe during pregnancy, and many have reported unexpected changes in their menstrual cycle after getting their shots. So, what’s the truth about Covid vaccines and pregnancy, fertility, and periods?

Covid vaccines and pregnancy—is there a risk?

Dr Patrick O’Brien, a London-based consultant obstetrician (OB/GYN) and VP at the UK’s Royal College of Obstetricians and Gynaecologists, explains that there is nothing in the vaccines that’s likely to be harmful during pregnancy, because they’re not live vaccines—that is, they don’t contain any of the live disease, so they can’t cause Covid. 

“We have decades of experience of giving ‘killed’ vaccines, like whooping cough or flu, in pregnancy and those have all been shown to be safe,” he says. “At the moment, all the evidence we have—and it's accumulating rapidly—is that these vaccines seem to be safe in pregnancy.”

Ariel Skelley

(Image credit: Ariel Skelley/Getty)

In the US, the vaccines were approved for pregnant people from the outset, and the American College of Obstetricians and Gynecologists (ACOG) “recommends that pregnant individuals have access to Covid-19 vaccines”. 

An ACOG spokesperson said, “While limited safety data specific to use of Covid-19 vaccines in pregnancy are available, to date there are no data to indicate that the vaccines should be contraindicated, and no safety signals were generated in studies. 

“Additionally, the evolving clinical data continues to demonstrate no safety signals among the thousands of pregnant women who have chosen to receive a Covid-19 vaccine.” They added that there is no preference for one Covid-19 vaccine over another.

As of last month, official guidance in the UK is that people who are breastfeeding or trying to conceive can have any of the available vaccines, while pregnant people should be offered the Pfizer or Moderna vaccines. 

That’s not to say that the other UK-approved vaccine (AstraZeneca) isn’t safe, but less data is available. “In the US, they have vaccinated 120,000 pregnant people with Pfizer and Moderna without any problems being flagged,” reproductive immunologist Viki Male says, “so that gives us confidence that those vaccines are safe in pregnancy”.

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(Image credit: John Fedele/Getty)

She adds that anyone who’s pregnant and has been vaccinated with a different vaccine should receive their second dose of the same one. Vaccination during pregnancy benefits babies, too, as New York OB/GYN Dr Daniel Roshan explains. “Some patients prefer to be vaccinated post-partum,” he says. “But the advantage of getting it during pregnancy is that you develop antibodies, and those antibodies are passed into the baby's circulation, so when they're born, they are also protected against Covid-19.”

Bear in mind that if you catch Covid-19 during pregnancy, you’re more likely to get severely ill and need hospital treatment, and at a higher risk of preterm birth. 

“Overall, it's better to take the vaccine than to get the disease, because at least with the vaccine we know that nobody has died from it and nobody has ended up in ICU,” Dr Roshan says. 

According to Dr O’Brien, there is “absolutely zero evidence whatsoever” to support claims of a link between Covid vaccines and fertility. 

“Separately, there isn’t any theoretical way and no plausible biological mechanism by which this could be the case,” he adds. 

Yeadon's original article didn’t actually state that vaccination causes female sterilization. He suggested that an element of the vaccine, called spike protein, is similar to a protein found in cells on the placenta, called syncytin-1. 

In his opinion, producing an immune response to the virus, the vaccine would have the unintended consequence of, "training the female body to attack syncytin-1, which could lead to infertility in women of an unspecified duration".

But according to authoritative sources—including those mentioned above—a glitch in the immune response is highly implausible. “The idea that our immune systems might attack the placenta after vaccination is very unlikely for lots of reasons,” says Male. 

She explains, “The two proteins are not similar enough that we would expect this to happen. If antibodies against spike did cause problems for the placenta, we would expect to see miscarriages in pregnant people who become infected with Covid-19, and we don’t see this.” 

What we do see is people successfully conceiving after they’ve been vaccinated. In the United States, almost 5,000 people have reported a pregnancy to the Centers for Disease Control and Prevention (CDC) following vaccination. That only accounts for people who knew they could or should report their pregnancy to the CDC, so in reality, the number of pregnancies is likely to be much greater.

Although pregnant people weren’t included in any vaccine trials, some trial participants did become pregnant accidentally. And they did so at the same rates whether they received the active vaccine or the placebo. 

“There have been IVF studies that, again, show that being vaccinated doesn’t make any difference to how likely patients are to become pregnant. All that really provides very strong evidence that the vaccine does not adversely affect fertility,” Male adds. 

Does the Covid vaccine affect your menstrual cycle?

The answer to this is more complex.

There’s plenty of anecdotal evidence of people experiencing changes to their periods—like bleeding more heavily, bleeding for longer, or their periods coming early or late—after having their Covid vaccination. 

A tweet by anthropology professor Dr Kate Clancy back in February asking whether others, like her, had noticed changes in their periods post-vaccination attracted so many responses that she and a former colleague created a survey tool to collect data. 

Westend61

(Image credit: Westend61/Getty)

They expected 500 participants. They got 100,000 within seven weeks. Other studies into a possible link are underway, too. We can’t say for sure yet whether changes to people’s periods are a direct side-effect of the Covid vaccination, because it wasn’t looked at during the initial clinical trials. But the fact that so many people say they’ve experienced menstrual changes—and there’s no reason to suspect any of them have an agenda—gives pause for thought, says Male. 

“Once we’ve studied this properly, I think having a slightly odd period after the vaccine will turn out to be a short-term side effect we warn people about, just like we tell people they might feel tired or feverish afterward,” she says. 

It wouldn’t be surprising if the Covid-19 vaccine caused such a response in some people—we can look to other types of vaccines for a pattern. 

For example, it’s been shown that the human papillomavirus (HPV) vaccine can potentially trigger slightly heavier or irregular periods in the short term—and importantly, without affecting fertility. 

Researchers looking at hormonal changes after the flu vaccine found a small reduction in progesterone, a hormone involved in menstruation. While they didn’t specifically examine the effect on periods, the lower hormone levels would tally with some temporary changes to periods. 

Can the vaccine make periods return after menopause?

It’s not just those who currently have a menstrual cycle, but also post-menopausal people, reporting unusual bleeding.

“There is some anecdotal evidence of irregular bleeding in both pre-menopausal and post-menopausal women,” says Dr Anne Henderson, consultant gynecologist, and British Menopause Society-accredited specialist. “The mechanism is slightly different, because one group of women is still ovulating and menstruating, and the other isn't.”

That could give scientists important clues as to how the vaccines might affect uterine bleeding. It’s also a good reason to properly investigate these symptoms during trials and research, like Dr Clancy’s and other studies are doing now. 

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As these symptoms haven't, to date, been included on vaccine information leaflets, it is causing some to panic.

Dr Henderson fears it could drive people on hormone replacement therapy (HRT) to stop taking their medication. “That can also cause sudden bleeding, which adds to the anxiety and it becomes a vicious circle,” she says. “If post-vaccination bleeding is a one-off event within the first four weeks or so, then I would reassure my post-menopausal patients that it’s nothing concerning.”

Male says, however, that until we have more clarity, anyone experiencing post-menopausal bleeding after their vaccination should see their doctor.

The verdict?

There is no evidence that any of the Covid-19 vaccinations affect fertility. There is good evidence that they don’t. 

Similarly, everything we know about the vaccines so far—and more than 1.5 billion doses have been given worldwide—tells us they are safe if you’re pregnant or breastfeeding. 

As for a link between vaccination and unusual uterine bleeding, more research is needed. Emerging evidence, together with data from other decades-old vaccines, suggest any changes to people’s normal menstrual cycle are short-term and nothing to worry about.