Pregnant women with COVID-19 face increased chances for stillbirths compared with uninfected women, and that risk spiked to four times higher after the delta variant emerged, new government data show.

The federal Centers for Disease Control and Prevention released a report today that examined 1.2 million deliveries in 736 hospitals nationwide from March 2020 through September 2021.

Stillbirths were rare overall, totaling 8,154 among all deliveries. But the researchers found that for women with COVID-19, about 1 in 80 deliveries resulted in stillbirth. Among the uninfected, it was 1 in 155.

Among those with COVID-19, stillbirths were more common in people with chronic high blood pressure and other complications, including those in intensive care or on breathing machines.

“These findings underscore the importance of COVID-19 prevention strategies, including vaccination before or during pregnancy,” CDC researcher Carla DeSisto and co-authors said.

There’s no information on how many had received COVID-19 shots, although the authors noted that the U.S. vaccination rate among pregnant people after delta emerged this past summer was 30%.

Pregnant people with COVID-19 are more likely than others to develop severe, even fatal, illness and they face increased risks for preterm birth and other complications. Previous studies on stillbirths and COVID-19 had mixed findings, but the report bolsters concerns among obstetricians and anecdotal data.

While the absolute risk for stillbirth is low, anyone who is pregnant shouldn’t underestimate the dangers of COVID-19, said Dr. Mark Turrentine, a professor at Baylor College of Medicine in Houston. He helped write the American College of Obstetricians and Gynecologists’ recommendations for COVID-19 vaccination in pregnancy.

“What’s really sad is we have 10 months of a vaccine that’s been highly effective and we just can’t convince people to take advantage of this,” Turrentine said.

Some experts have speculated that the virus may cause inflammation in the placenta or other abnormalities that could harm the fetus.

Dr. Joseph Biggio, a specialist in high-risk pregnancies at Ochsner Health in New Orleans, said the study doesn’t prove that COVID-19 caused stillbirths. He said it’s possible some women were so critically ill that physicians trying to keep them alive “couldn’t intervene on behalf of a fetus that they knew was in trouble.”

The researchers relied on medical records, and they noted that they were unable to determine if the COVID-19 diagnoses listed at the time of delivery represented current or past infections.

Generally, stillbirths are more common among Black people, those who become pregnant over age 35 or those who smoke tobacco during pregnancy.

The study didn’t include pregnancy outcomes by race, an area the authors said they plan to investigate in future research “because COVID-19 has disproportionately affected many racial and ethnic minority groups, putting them more at risk of getting sick and dying.”