A C.D.C. study finds stillbirths are higher in pregnant women with Covid.

Pregnant women who had Covid-19 when they delivered their babies were almost twice as likely to have a stillbirth as healthy women who did not have Covid, according to a new Centers for Disease Control study released Friday that examined more than 1.2 million deliveries in the U.S. between March 2020 and September 2021.

While stillbirths were rare overall, representing fewer than 1 percent of all births, 1.26 percent of the 21,653 women with Covid experienced a stillbirth, compared with 0.64 percent of women without Covid. Even after adjustments were made to control for differences between the groups, women with Covid were 1.9 times as likely as healthy women to have a stillbirth.

The risk of stillbirth has been even higher for women with Covid since the Delta variant has been dominant: while the risk of stillbirth for women with Covid was 1.5 times as high as that of healthy women before July, when Delta became dominant, it was 4 times as high between July and September. As many as 2.7 percent of deliveries to women with Covid were stillbirths during the period studied while Delta was dominant.

“There had been reports suggesting an increased risk, but stillbirths are hard to study, because luckily they are uncommon,” said Dr. Denise Jamieson, chief of gynecology and obstetrics at Emory Healthcare. “This is some of the strongest evidence of the increased risk, and probably the strongest data pointing to the risks specifically tied to Delta.”

“The take home message is the importance of vaccination for pregnant women, particularly at this point in the pandemic with Delta circulating,” she said.

The C.D.C. strongly encourages pregnant and breastfeeding women and women planning or trying to become pregnant to be vaccinated against Covid, but resistance has been strong, even though pregnancy is on the C.D.C.’s list of health conditions that increase the risk of severe disease.

Studies have shown that pregnant patients who are symptomatic are more than twice as likely as other symptomatic patients to require admission to intensive care or interventions like mechanical ventilation, and they may be more likely to die. They are also more likely to experience a preterm birth.

Another C.D.C. study issued Wednesday described the cases of 15 pregnant women in Mississippi who died of Covid during their pregnancy or shortly afterward, including six who died before the Delta variant became dominant and nine who died between July and October, while Delta was dominant.

Of the women who died, nine were Black women, three were white women and three were Hispanic women. The median age was 30. Fourteen of the women had underlying medical conditions, and none were vaccinated. Five of the deaths occurred before vaccinations were available.

The study concluded that the risk of death for a pregnant woman with Covid is nine deaths per 1,000 infections, while the risk of death from Covid for other women of reproductive age is only 2.5 deaths per 1,000 infections.

A third study by C.D.C. researchers found that pregnant women with Covid faced a more than 60 percent higher risk of being admitted to intensive care, needing a ventilator or special equipment to breathe, and even of dying during the period that Delta was dominant, compared to pregnant women during the period before the variant was dominant.