Pregnant women will not be a part of the first two COVID-19 vaccine trials that are occurring in the U.S. as drugmakers race to develop a vaccine to treat the virus, researchers told Reuters. The absence of pregnant women from the studies raises concerns over how childbearing women will be protected from COVID-19.

Both clinical trials conducted by Moderna and Pfizer, along with BioNTech, are requiring proof of a negative pregnancy test as well as the use of birth control for those who enroll as the companies look to ensure the vaccine is safe for mass-market production, the news outlet said.

To move the use of vaccine safely to pregnant women, U.S. regulators reportedly require safety testing on pregnant animals to make sure they don’t pose a risk to fetuses or cause miscarriages. The idea of eliminating pregnant women from vaccine trials has come with criticism, especially as studies have shown that this population has an increased risk of contracting the coronavirus.

“It’s a problem because if (vaccines) are not tested in pregnancy, then they may not be available or people may not be comfortable offering them,” Dr. Denise Jamieson, chief of gynecology and obstetrics for Emory Healthcare in Atlanta told Reuters.

COVID-19 vaccine developer Modera told the news outlet that it has started its studies in pregnant animals at the end of June and expects results by the end of 2020.

“Once we have generated additional safety data for our vaccine, and importantly demonstrated that it is efficacious, we intend to conduct additional studies in this important population,” a Moderna spokesperson told Reuters.

Sanofi is also reportedly doing reproductive toxicology in animals, but results will not be ready before the start of its Phase 3 trials, which are slated to start by year-end while Merck said it has not made a decision on when to test pregnant women with the vaccine. Both Novavax and AstraZeneca have not said what their current plans are for pregnant women and a COVID-19 vaccine.

As many as 76.8 million women are of childbearing age from 15 to 50 years old, the 2012 Census said.

The U.S. has reported over 4.49 million positive cases of the coronavirus with more than 17.3 million confirmed cases worldwide as of late morning on Friday, Johns Hopkins University said. There have been over 152,000 COVID-19 deaths in the U.S. and over 674,000 COVID-19 deaths in the world, the university said.