New U.S. medical data shows that pregnant women with COVID-19 face increased chances for stillbirths, and that association gets stronger during the period of Delta variant predominance, according to two studies released Friday by the U.S. Centers for Disease Control and Prevention (CDC).

One report found that 15 pregnant patients in Mississippi died of COVID-19-related causes between March 2020 and early October 2021, including nine who died after the Delta variant became dominant. All but one of the women who died had underlying health conditions, and none had been fully vaccinated. 

The second report, which examined 1.2 million deliveries in 736 hospitals nationwide from March 2020 through September 2021, found that the risk of stillbirth increased about fourfold for women with COVID-19 as Delta surged.

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

Overall, stillbirths were highly rare, totaling 8,154 among all deliveries, and were more common in people with chronic high blood pressure and other complications, including those in intensive care or on breathing machines.

The report didn't include information on how many had received COVID-19 shots, although the authors noted that the U.S. vaccination rate among pregnant people was approximately 30 percent as of July 2021.

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 report was based on medical records, and researchers acknowledged 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 mothers compared to Hispanic and white mothers, according to the CDC. 

The study on COVID-10-related stillbirths, however, didn't include pregnancy outcomes by race, as previous data has shown that COVID-19 has disproportionately affected many racial and ethnic minority groups, putting them at high risk of virus exposure. 

(With input from agencies)