Pfizer and BioNTech have started an international study with 4,000 volunteers to evaluate the safety and effectiveness of their COVID-19 vaccine in healthy pregnant women, the companies said on Thursday.
Drugmakers have said they first need to make sure the vaccines are safe and effective more generally.
Pregnant women in the United States have already received their first doses, the companies said.
The new study will test pregnant women aged 18 and older in the United States, Canada, Argentina, Brazil, Chile, Mozambique, South Africa, the UK and Spain.
Women will receive the vaccine during weeks 24-34 of gestation, getting two shots 21 days apart - the same regimen used in the larger clinical trial. Shortly after giving birth, participants who got a placebo in the trial will be given an opportunity to get the actual vaccine, while remaining part of the study, the companies said.