A study published in the journal The Lancet Infectious Diseases revealed that the mRNA COVID-19 vaccines are effective at reducing the risk of severe illness in pregnant people and the risk of COVID-19 hospital admission. The study was conducted by Dr. Manish Sadarangani and colleagues between December 2020 and November 2021. In the study, the cases from the Canadian National Vaccine Safety (CANVAS) Network reporting adverse events within a week following COVID vaccination were selected. The study compared the adverse event data among pregnant people with that of both unvaccinated pregnant people and vaccinated non-pregnant females.

The study showed that 4%,i.e., 226 vaccinated pregnant people out of 5597, reported a significant health event, like a new or worsening health event sufficient to cause work or school absenteeism, medical consultation, or prevent daily activities after dose 1 of the primary series mRNA COVID-19. Similar adverse events were seen for both mRNA-1273 [Moderna] and BNT162b2 [Pfizer BioNTech] vaccinations.

Additionally, 227 vaccinated pregnant women out of 3108 reported a significant health event after dose 2 of a primary series. However, adverse events differed by vaccine type in this 7.3% of vaccinated pregnant women. 12.1% of the pregnant individuals reported a significant health event after dose 2 of mRNA-1273[Moderna]. On the other hand, only 4.2% of the cases were reported for BNT162b2 [Pfizer BioNTech] vaccination. A multivariable analysis in the study revealed that among pregnant people immunized with either of the vaccines, pregnancy was associated with a reduced risk of significant adverse events in the week following each dose of the jab. 

(Source: https://www.livemint.com/science/health/mrna-vaccines-safe-during-pregnancy-says-lancet-report-11660264277893.html )