As the delta variant of COVID-19 spread across the United States this summer, the virus appeared to take a particular toll on unvaccinated pregnant people, with deaths dramatically increasing in the summer months.
The number of pregnant people who died of COVID-19 spiked sharply in August and September, with more than two dozen deaths recorded in each of those months, according to data released this week by the U.S. Centers for Disease Control and Prevention (CDC).
More than 40% of the 248 deaths among pregnant people since the start of the pandemic occurred since August, the data shows.
The number of pregnant people who contracted COVID-19 also increased sharply over the summer months, according to CDC data, reaching numbers of cases not seen since before the vaccine was made widely available earlier this year.
Now, as the omicron variant spreads across the U.S., with what is believed to be a high degree of transmissibility, the director of the CDC said she is "very concerned" about those who remain unvaccinated.
"I can tell you, when I hear about a pregnant woman in the community who is not vaccinated, I personally pick up the phone and talk to them," CDC Director Dr. Rochelle Walensky told ABC News' chief medical correspondent, Dr. Jennifer Ashton, in a Dec. 8 interview.
"It's just shocking," she said of the number of pregnant people who died specifically in August, one month after the delta variant became the predominant variant in the U.S.
More than 25,000 pregnant people have been hospitalized since the start of the pandemic, and more than 150,000 cases of COVID-19 have been confirmed in pregnant people, according to the CDC.
Pregnancy is included in the CDC's list of underlying medical conditions that make a person more likely to experience severe illness from COVID-19.
The virus causes a two-fold risk of admission into intensive care and a 70% increased risk of death for pregnant people, and increases the risk of a stillbirth or delivering preterm, or earlier than 37 weeks, according to the CDC.
COVID-19 is especially dangerous in pregnant people because their immune systems are already less active as they are supporting their growing fetus. For the same reason, their hearts and kidneys are working harder, Dr. Laura Vricella, a maternal fetal medicine physician at Mercy Hospital in St. Louis, told ABC News in August, as her hospital and others experienced a spike in pregnant patients with COVID-19.
Pregnant people must also keep their oxygen levels higher in general to support their fetus, which can be a herculean task to do when COVID-19 is in the body, according to Vricella.
And in addition to pregnant people with COVID-19 being more likely to deliver prematurely, Vricella said her hospital also saw more COVID-positive pregnant patients deliver stillbirths, even with mild COVID cases.
MORE: Families speak out as COVID-19 delta variant proves dangerous for pregnant people"COVID-19 begins as a respiratory illness, but can affect the entire body and also seems to increase the risk of thrombosis or blood clots," she said. "We suspect that this decreased oxygen to the fetus may be responsible for the stillbirths that we are seeing, though we need further research."
In September, the CDC issued an "urgent health advisory" calling on pregnant people to prioritize getting vaccinated against the virus.
As of Dec. 4, the most recent data available, the vaccination rate among pregnant people remains below 40%, compared to nearly 61% of the general population, according to the CDC.
MORE: Nurse who lost her baby after contracting COVID-19 urges pregnant women to get vaccinatedThe vaccination rate for Black pregnant people, who already face disproportionate health risks in pregnancy and postpartum, is even lower, at just over 20%, CDC data shows.
"This is one where I feel like we have to do more," Walensky said of the low vaccination rate overall among pregnant people. "We have to do better."
"The vaccines are safe, they are effective and they are even more important in pregnant women," she said.
In addition to the CDC, the nation's two leading health organizations focused on the care of pregnant people -- American College of Obstetricians and Gynecologists (ACOG) and the Society for Maternal-Fetal Medicine (SMFM) -- have issued guidelines calling on all pregnant people to get vaccinated against COVID-19.
Though pregnant people were not recruited for the initial clinical trials of the COVID-19 vaccines, data over the past several months, since vaccines have been widely available, has shown them to be safe for pregnant people.
In its health advisory urging pregnant people to get vaccinated, the CDC pointed specifically to new data showing the vaccines did not increase the risk of miscarriage. The vaccines are also not believed to have any "significant impact" on fertility.
Both the Pfizer and Moderna vaccines use mRNA technology, which does not enter the nucleus of the cells and doesn't alter the human DNA. Instead, it sends a genetic instruction manual that prompts cells to create proteins that look like the virus, as a way for the body to learn and develop defenses against future infection.
They are the first mRNA vaccines, which are theoretically safe during pregnancy because they do not contain a live virus.
Vaccine experts interviewed by ABC News said although pregnant women are advised against getting live-attenuated virus vaccines, such as the one for measles, mumps and rubella, because they can pose a theoretical risk of infection to the fetus, the Johnson & Johnson vaccine doesn't contain live viruses and should be safe. Instead the Johnson and Johnson vaccine uses inactive viruses.
Health experts said that with or without the vaccine, pregnant people need to continue to remain on high alert when it comes to COVID-19 by following safety protocols, including mask wearing, social distancing and hand washing.