evidence: Again, the scientific data from the large phase 3 trials of the Pfizer-BioNTech and Moderna Covid-19 vaccines demonstrated high protective efficacy when the second dose was the same vaccine as the first dose. These vaccines are not interchangeable and the safety and efficacy of combinations of vaccines have not been evaluated. However, the Centers for Disease Control and Prevention recently stated that, in exceptional situations in which the first-dose vaccine product cannot be determined or is no longer available, any available mRNA Covid-19 vaccine may be administered to complete the mRNA Covid-19 vaccination series. [...] Although the large phase 3 trials of the Pfizer-BioNTech and Moderna Covid-19 vaccines did not include pregnant women, eligible pregnant women may be vaccinated. The two mRNA vaccines are not contraindicated during pregnancy. Pregnant women should consider the risks of Covid-19, which may be more severe in pregnant women, and the uncertain risk of vaccination. More information on the safety and efficacy of Covid-19 vaccines in pregnant women will become available over time.