A new study has added to the body of research showing the risks unvaccinated pregnant people face amid the coronavirus pandemic.
Unvaccinated pregnant people with COVID-19 and their newborn babies have a higher risk of complications from the disease, like hospitalizations and newborn deaths, compared to those who are vaccinated, according to the study, published Thursday in Nature Medicine.
The study, led by researchers in Scotland, found that unvaccinated pregnant people who contracted COVID not only had a risk of more severe illness themselves, but also were more likely to experience pregnancy loss or preterm birth compared to other women.
Like the United States, Scotland is also facing a low vaccination rate among pregnant people, leading the researchers to conclude, "Addressing low vaccine uptake in pregnant women is imperative to protect the health of women and babies."
As of Jan. 8, around 40% of all pregnant people ages 18-49 years-old in the U.S. have been fully vaccinated, prior to or during pregnancy, according to the Centers for Disease Control and Prevention, compared to over 70% of the general population.
In December, the director of the CDC, Dr. Rochelle Walensky, told ABC News in an interview she was "very concerned" about those who remain unvaccinated, particularly pregnant people.
"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," said Walensky, who led the CDC in issuing an "urgent health advisory" in September calling on pregnant people to get vaccinated.
Since the start of the pandemic, the CDC reports there have been more than 150,000 confirmed cases of COVID-19 in pregnant people, including nearly 26,000 hospitalizations and 257 deaths.
Here is what pregnant and breastfeeding people may want to know about the COVID-19 vaccines to help them make informed decisions.
1. When can pregnant people get a COVID-19 vaccine?
Everyone 12 years of age and older, including pregnant people, is now eligible to get a COVID-19 vaccination, according to the CDC.
Pregnant people can get the COVID-19 vaccine at any point in their pregnancy and while breastfeeding, and the vaccine does not need to be spaced from other vaccines, like the flu shot or Tdap booster.
2. What is the science behind the COVID-19 vaccine?
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 part of 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.
The Johnson & Johnson vaccine uses an inactivated adenovirus vector, Ad26, that cannot replicate. The Ad26 vector carries a piece of DNA with instructions to make the SARS-CoV-2 spike protein that triggers an immune response.
This same type of vaccine has been authorized for Ebola, and has been studied extensively for other illnesses -- and for how it affects women who are pregnant or breastfeeding.
The CDC has concluded that pregnant people can receive the Johnson & Johnson one-shot vaccine after reviewing more than 200 pages of data provided by the company and the U.S. Food and Drug Administration (FDA).
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 virus and should be safe.
3. Are there studies on pregnant women and the COVID-19 vaccine?
Yes, studies continue to show the COVID-19 vaccine is safe and effective for pregnant people.
A study released Jan. 4 by the Centers for Disease Control and Prevention (CDC), found no increased risk of preterm or low-weight birth among babies born to pregnant people who got a COVID-19 vaccine shot, compared to babies born to unvaccinated pregnant people.
The study's researchers, at Yale University, looked at the health data of more than 40,000 pregnant people and did not identify any safety issues with getting vaccinated while pregnant, no matter which trimester a woman was vaccinated, or how many vaccine doses she got during her pregnancy. Researchers noted most of the women included in the analysis were vaccinated in the second or third trimester, and the study didn't include booster doses.
In a health warning issued in September urging pregnant people to get vaccinated, the CDC said data shows there is also no increased risk for miscarriage linked to receiving a COVID-19 vaccine.
"Miscarriage rates after receiving a COVID-19 vaccine were similar to the expected rate of miscarriage," the CDC said at the time. "Additionally, previous findings from three safety monitoring systems did not find any safety concerns for pregnant people who were vaccinated late in pregnancy or for their babies."
In addition, two studies released last summer found Pfizer and Moderna's COVID-19 vaccines appear to be safe and effective for pregnant people, and were also found to likely offer protection as well to infants born to a vaccinated person.
One study, published May 11 in the journal Obstetrics & Gynecology, is believed to be the first to examine the impact of the COVID-19 vaccines on the placenta, according to the university. Researchers found the vaccine had no impact on pregnancy and no impact on fertility, menstruation and puberty.
A second study, led by researchers at Beth Israel Deaconess Medical Center and Harvard Medical School, looked at more than 100 women who chose to get either the Pfizer or Moderna vaccines and found that the women's antibodies against COVID-19 after being fully vaccinated were also present in infant cord blood and breast milk.
An earlier study, a study published in the American Journal of Obstetrics & Gynecology in March found the Pfizer and Moderna vaccines are safe and effective in pregnant and lactating people and those people are able to pass protective antibodies to their newborns.
4. What are health groups saying about the COVID-19 vaccine?
In August, the CDC strengthened its recommendation for COVID-19 vaccination during pregnancy, citing new evidence of safety with the vaccines.
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) -- also issued new guidelines calling on all pregnant people to get vaccinated against COVID-19.
In their joint recommendation issued in July, ACOG and SMFM said pregnant people should "feel confident" in getting vaccinated against COVID-19.
"ACOG is recommending vaccination of pregnant individuals because we have evidence of the safe and effective use of the vaccine during pregnancy from many tens of thousands of reporting individuals, because we know that COVID-19 infection puts pregnant people at increased risk of severe complications, and because it is clear from the current vaccination rates that people need to feel confident in the safety and protective value of the COVID-19 vaccines," ACOG president Dr. J. Martin Tucker said in a statement. "Pregnant individuals should feel confident that choosing COVID-19 vaccination not only protects them but also protects their families and communities."
"COVID-19 vaccination is the best method to reduce maternal and fetal complications of COVID-19 infection among pregnant people," Dr. William Grobman, president of SMFM, said in a statement announcing the new recommendation, also noting the vaccines are safe before, during and after pregnancy.
The World Health Organization (WHO) also says pregnant people can be vaccinated against COVID-19.
"Limited data are currently available to assess the safety of COVID-19 vaccines in pregnancy. However, based on what we know about the kinds of vaccines being used, there is no specific reason for concern," WHO says on its website. "None of the COVID-19 vaccines authorized to date use live viruses, which are more likely to pose risks during pregnancy."
5. What will clinical trials be like for pregnant people?
Pfizer's phase 2/3 trial will enroll approximately 4,000 women within weeks 24-34 of their pregnancy, the company announced in a press release.
Half will get the vaccine, and half will get a placebo.
The study will include healthy, pregnant woman age 18 and older in the U.S., Canada, Argentina, Brazil, Chile, Mozambique, South Africa, the United Kingdom and Spain.
Participants in the vaccine group will receive two doses at 21 days apart -- and each woman will be followed for at least 7-10 months in order to continuously assess for safety in both participants and their infants.
Infants will also be assessed, up until 6 months of age, for transfer of protective antibodies from their vaccinated mother.
Women enrolled in the trial will be made aware of their vaccine status shortly after giving birth to allow those women who originally received placebo to be vaccinated while staying in the study.
6. Why weren't pregnant people included in early clinical trials?
Not recruiting parents-to-be in clinical trials and medical research is nothing new, according to Dr. Ruth Faden, the founder of the Johns Hopkins Berman Institute of Bioethics and a bioethicist who studies the ethics of pregnancy and vaccines.
"For a very long time, pregnant women were not included in biomedical research evaluation efforts or clinical trials, both for concerns about fetal development and what would be the implications of giving a pregnant women an experimental drug or vaccine and also for legal liability worries from manufacturers and pharmaceutical companies," Faden told "GMA" last month. "There's a huge gap between what we know about the safety and effectiveness of a new drug or a new vaccine for the rest of the population and what we know about it specific to pregnancy."
In the case of the COVID-19 vaccines, health experts have only one of the three sources of evidence that are used to evaluate safety and efficacy during pregnancy: the data on non-pregnant people who were enrolled in the clinical trials, according to Faden.
From that, Faden said, health experts can try to glean what side effects may happen to people who are pregnant, but it is not an exact science.
However, it's considered typical -- and many argue ethically appropriate -- to study an unknown substance first in healthy adults and then progressively in broader and broader populations. Pregnant people and children are often tested later down the line because of concerns about potential long-term harm.
Some of the volunteers in prior COVID-19 vaccine trials that didn't include pregnant women directly may still become pregnant during the trial. This will also give researchers some insights about the vaccine's safety among this group.
7. Is COVID-19 more dangerous for pregnant people?
According to the CDC, COVID-19 causes a two-fold risk of admission into intensive care and a 70% increased risk of death for pregnant people.
Health experts say 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 face mask wearing, social distancing and hand washing.
ABC News' Sony Salzman, Arielle Mitropoulos, Siobhan Deshauer and Eric Strauss contributed to this report.
Study finds risk of severe illness, stillbirths for unvaccinated pregnant people