Pregnant women who received at least one dose of a COVID-19 vaccine were not at higher risk for miscarriage than their unvaccinated counterparts, according to a new study.

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In the study, published in JAMA, researchers from the Vaccine Safety Datalink (VSD) collaboration between the Centers for Disease Control and Prevention (CDC) and nine health systems examined 105,446 women from December 15, 2020, to June 28, 2021. Of these, 13,160 ended up in miscarriages, while 92,286 pregnancies were ongoing.

Among the women, 14.3 per cent received at least one dose of the Pfizer, Moderna or Johnson & Johnson vaccine. The team looked at women between six weeks' and 19 week's gestation and identified the 'index date' as the last day of the four-week surveillance period. They found that women had received a COVID-19 vaccine 28 days before the index date in 8 per cent of ongoing pregnancies and 8.6 per cent of miscarriages, the Daily Mail reported.

Further, the proportion of women aged 35 to 49 years who miscarried was higher than the proportion with ongoing pregnancies in that age-group (38.7 per cent vs 22.3 per cent). Overall, this means women who suffered miscarriages did not have higher odds of receiving a COVID shot in the previous 28 days compared to women who did not suffer miscarriages, the researchers explained.

"Despite limitations, these data can be used to inform vaccine recommendations and to counsel patients," said Heather S. Lipkind, Obstetrics and Gynecology, Yale University in Connecticut. The findings come about a month after the CDC urged pregnant women to receive the COVID-19 vaccine as soon as they can.

In a separate study, a team from the CDC looked at 2,456 pregnant women who received either the Pfizer-BioNTech or the Moderna COVID vaccine prior to 20 weeks' gestation as of July 19, 2021, from the agency's smartphone tool V-SAFE that tracks people who received COVID shots.

The study, published on pre-print server Research Square, also looked at the risk of miscarriage, also described in the study as a 'spontaneous abortion', the Daily Mail reported. While miscarriages occured in 11-16 per cent of pregnancies, the rate of miscarriage in women who received the COVID shot was 12.8 per cent within the normal range.

Further, the miscarriages ranged from 9.8 per cent among 20-to-29-year-olds to 28.8 per cent in those aged 40 and older. This is also in line with data, which find that women are at a higher risk for a miscarriage when they conceive at age 35 or older, the report said.