In recent weeks, pregnant women have been prioritized for coronavirus vaccinations across the country.

Washington, D.C. and 21 states — most recently including Ohio, Iowa and South Carolina — have added pregnancy to their lists of eligible high-risk conditions, according to NBC News' research. The Centers for Disease Control and Prevention considers Covid-19 an acute threat during pregnancy. Six states are not following the CDC's recommendation to prioritize pregnant women over the general population.

As states move forward with plans to add pregnant women to their eligibility lists, Pfizer-BioNTech has begun a clinical trial of their vaccine's effect on pregnant women to supplement the lack of data about pregnancy and the vaccines.

Pregnant women have no biological reason for concern, said Dr. Brenna Hughes, vice chair of obstetrics and quality at Duke Medical. "Based on the biologic mechanism of vaccines, we can't see a biologic reason it would be unsafe."

All pregnant women are at a greater risk of complications if exposed to the coronavirus and should be vaccinated as soon as they are eligible, no matter how far along in their pregnancies they are or if they are breastfeeding, Hughes said.

The CDC recommends that all states include pregnant women in its phase 1C of vaccinations; not all states are following the guidelines, and some may require pregnant women to have doctor's notes or verification of their pregnancies before they get vaccine shots.

Many states recommend, some on their websites, that those eligible with high-risk health conditions, such as pregnancy, consult with their doctors before they schedule their vaccination appointments.

In New Mexico, those seeking vaccinations may be asked to verify any health conditions, pregnancy included, before they get shots. In New York, you must have proof of your current eligibility, which may be a doctor's letter, medical information with evidence of underlying condition or a signed certification. Florida requires a note from a doctor, and those in Washington, D.C. must show proof that they meet the eligibility criteria.