Pregnant women who contract mild to severe COVID-19 are about three times more likely to develop complications leading to death or severe illness than uninfected mothers, a new study has found. 

Researchers analyzed data on more than 13,000 pregnant women at 17 U.S. hospitals from March 1 to Dec. 31, 2020, before COVID-19 vaccines were widely available. About 2,350 of the women had been infected by COVID-19. 

Twenty-six percent of mothers in the study who had mild to severe COVID-19 died or had serious illnesses, compared to 9 percent among those who avoided the virus. All five women who died had tested positive for the virus. 

Mothers with mild to severe infections also were 40% more likely to deliver by cesarean section and nearly twice as likely to deliver prematurely, and their babies were twice as likely to die in the womb or as newborns, the study found.

Serious complications in infected mothers included seizures, severe high blood pressure, kidney failure and other organ damage caused by high blood pressure, sepsis from infections other than SARS-CoV-2, and endometritis requiring prolonged use of antibiotics, according to the researchers.

There was no link found between mild or asymptomatic cases of COVID-19 and pregnancy complications.

The study was led by Dr. Torri Metz, an obstetrician at University of Utah Health, and funded by the National Institutes of Health. It was published Monday in the Journal of the American Medical Association.

The researchers say the findings underscore the need for women who are pregnant or may become pregnant to get vaccinated and take other precautions to prevent against COVID-19 infection.

“The complications of pregnancy we observed were mostly in people who had moderate to severe symptoms of COVID-19,” Metz said in a news release. “We know from other studies that vaccination prevents the most severe symptoms of the disease. So, this is just another piece of the puzzle that should encourage pregnant people to get vaccinated.”

Surveys have found that reproductive-aged adults have cited concerns about potential side effects of COVID-19 vaccines on fertility and pregnancies for not getting immunized. But Metz’s study is among a handful in recent weeks that point to the virus itself being the far greater concern.

A study out of Scotland published last month found the rates of baby deaths and premature births were higher among mothers who were infected with the coronavirus 28 or fewer days before their delivery date. 

And a Boston University study found no significant difference in conception rates between unvaccinated couples and couples in which at least one partner had received at least one vaccine dose. The researchers, however, did observe that couples with male partners who tested positive for COVID-19 within 60 days of a given menstrual cycle were 18% less likely to conceive. 

Early research has found no evidence that the Pfizer and Moderna mRNA vaccines cause any serious side effects in pregnant women, and the CDC recommends anyone who is pregnant, is breastfeeding or might become pregnant in the future to get vaccinated.

Vaccines can be administered at any time during pregnancy.