What we know about Covid-19 vaccines and miscarriages

19 August 2021

We’ve seen a number of claims over the last few months about the Covid-19 vaccines and miscarriages. Here we debunk a few of the most common ones.

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What we know so far

There is no scientific reason the vaccine might be expected to affect pregnancy. A new, as yet not peer-reviewed, study looking at 2,456 pregnant women who got an mRNA vaccine (such as those from Moderna or Pfizer) before they conceived or up to 20 weeks found there was no increased chance of miscarriage.

We do know from several studies that severe Covid illness is more likely for women who are pregnant than those who aren’t, especially for those in the third trimester

The decision to offer pregnant women the Covid-19 vaccines is based on real-world data from the US, where 130,000 pregnant women have been vaccinated, mainly with mRNA vaccines including Pfizer-BioNTech or Moderna vaccines, without any safety concerns being raised.

A study hasn’t proved 82% of pregnancies end in miscarriage

A claim we’ve seen circulating around miscarriages and the vaccine was that a study had shown 82% of pregnancies end with miscarriage following Covid vaccinations. This is wrong.

This figure is based on an incorrect calculation. The study in question followed almost 4,000 women who had been pregnant when they got their vaccination. 

When the researchers published their study, a lot of the participants were still pregnant. So they only looked at the 827 completed pregnancies, meaning those that had ended in a live birth, still birth, abortion or miscarriage.

Of those, 104 sadly ended in miscarriage, or about 12.6% of completed pregnancies. This is similar to the proportion of pregnancies sadly expected to end in miscarriage regardless of vaccine status.

The calculation resulting in the false 82% figure looked at how many miscarriages occurred in the relatively few (127) who were vaccinated in their first or second trimesters. But the study was only looking at completed pregnancies. While the researchers said the data did not indicate “obvious safety signals”, they added that “more longitudinal follow-up, including follow-up of large numbers of women vaccinated earlier in pregnancy, is necessary”.

Viki Male, an immunologist working on pregnancy at Imperial College London, called it “a meaningless statistic” on Twitter, since it is not possible to complete a pregnancy in only three months and have it result in a live birth. 

She wrote: “And presumably the only reason that rate isn't 100% is because some people vaccinated at the end of the 2nd trimester completed their pregnancies within 3 months by giving birth.

“So, from the data in this paper we can calculate the miscarriage rate on completed pregnancies as 12.6%. Or we can calculate the miscarriage rate on (probably) completed first trimesters as 8.5%. Both of these are no higher than normal. 

“But the quoted rate of 84% is not the miscarriage rate. It just tells us that we can't go from the first trimester to giving birth to a healthy baby in three months. Which I think we all knew, right?”

Be wary of reports of miscarriage figures after vaccination from the Yellow Card scheme

We’ve also been asked about claims that miscarriages increased by 3,016% “as a result of” the Covid vaccine. Again, this is wrong.

It’s based on data from the Medicines and Healthcare products Regulatory Agency’s Yellow Card scheme, which collects reports of adverse events following vaccination, but does not prove that vaccination was the cause of any of them.

This 3,000% figure appeared after the first summary of Yellow Card data (covering 9 December 2020 to 24 January 2021) showed six reported miscarriages (recorded as spontaneous abortions) which increased to over 170 for the Pfizer and AstraZeneca vaccines as of Yellow Card data up to 9 June. 

It is sadly to be expected that the number of miscarriages reported after vaccination has increased, as the number of women being vaccinated increased and younger women were invited to be vaccinated. Women aged over 45 were first invited to be vaccinated in mid-April, and over-30s were invited in late May. 

Background levels of miscarriage

If you see a claim about miscarriages being caused by the Covid vaccine, it’s important to remember that a certain number of miscarriages are unfortunately expected in any group of pregnant women.

The number of miscarriages isn’t officially recorded, but Tommy’s National Centre for Miscarriage Research estimates that one in eight pregnancies ended in miscarriage, where the mother knew they were pregnant and subsequently miscarried. It estimates one in five pregnancies in total end in miscarriage.

The information included in this article contains the latest evidence and official guidance available at the time it was written. This is not a substitute for medical advice. If you require specific medical advice please consult your GP or midwife.

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