The two studies, conducted by the C.D.C. from March 1 through Aug. 22, followed 598 pregnant women hospitalized in 13 states.

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Pregnant women who are infected with the coronavirus and hospitalized are at risk for developing serious complications, and may face an elevated risk for delivering their babies prematurely, according to new studies from the Centers for Disease Control and Prevention. They may also be at greater risk of losing the pregnancy or having a stillbirth.

The troubling findings are consistent with some earlier reports that pregnant women may be at increased risk for severe illness when they become infected with the new coronavirus. But some experts warned that the findings, drawn from relatively small numbers of patients, including many hospitalized because of Covid-19, may not be representative of all pregnant women who are infected.

The studies found that many hospitalized pregnant women who were infected with the virus did not have any symptoms. Among those who did have symptoms, however, between 16 percent and 30 percent required intensive care, and 6 percent to 8.5 percent required ventilators to help with breathing. Among the 703 cases described in the two reports, three of the women died.

Both studies found that pregnant women infected with the coronavirus experienced a higher rate of preterm deliveries than expected, and some had stillbirths. Earlier studies have also suggested a higher risk for preterm births, and a British study noted a population-wide uptick in stillbirths during the pandemic.

Pregnancy may make women more vulnerable to infection and severe illness for several reasons. The immune system is suppressed during pregnancy, a response designed to prevent adverse reactions to the fetus but one that increases the mother’s susceptibility to viral infections.

Other physiological changes during pregnancy may also increase women’s vulnerability. The lungs may be affected by the expanding uterus, and the cardiovascular system is working harder. Covid-19 can also raise the risks of blood clots, and little is known about effects on the placenta, which nourishes the fetus.

“We now have data from three separate C.D.C. surveillance systems all suggesting that pregnant women may be at increased risk for severe disease from Covid-19,” Dr. Denise Jamieson, a member of the Covid-19 task force at the American College of Obstetricians and Gynecologists, said in an email.

“The take home message is that pregnant women can get seriously ill with this,” said Dr. Peter Bernstein, director of the division of maternal fetal medicine at Montefiore Medical Center in the Bronx. “We don’t know for sure that they will get sicker than they would have if they weren’t pregnant, but certainly there are women out there who are getting very sick and even dying.”

Although experts said that more research is needed to clarify these links, they urged pregnant women to be scrupulous about wearing masks and social distancing in order to minimize the risk of infection, especially if they have underlying health problems or conditions such as obesity. Some called for screening all pregnant women for the coronavirus, whether they have symptoms or not.

Dr. Neel Shah, an assistant professor of obstetrics and gynecology at Harvard University, said pregnant women should be prioritized for testing, and he called on employers to take extra measures to protect them from exposure to the virus while working.

“If there is one call for action, it is that employers need to take care of their pregnant people, especially if they’re getting close to term, and do everything possible to avoid them being exposed — allowing them to work virtually or giving the time and space they need,” Dr. Shah said.

But he said the studies, while providing “a signal,” that requires further investigation, are not definitive.

Dr. Shikha Garg, a medical epidemiologist at the C.D.C. and senior author of one of the studies, said an important finding was that so many pregnant women who had no symptoms of Covid-19 were infected with the virus.

“We’re still learning about how Covid-19 may affect pregnant women and their newborns,” Dr. Garg said. “If testing policies just focus on symptomatic women, we may miss the asymptomatic ones.”

The new reports came from two different C.D.C. surveillance systems.

One study looked at 598 pregnant women with Covid-19 hospitalized in 13 states from March 1 through Aug 22. Like most pregnant women, their median age was young — 30 — but one in five had a chronic health problem, most commonly asthma or hypertension. About 42 percent were Hispanic and 26.5 percent were Black.

More than half of these women were asymptomatic when they were admitted, for a variety of reasons, to the hospital. Among the 272 pregnant women who had symptoms, 16 percent required intensive care, and 8.5 percent required ventilators to help with breathing. Two women died.

Of the entire group of 598 women in this study, 458 completed their pregnancies during the hospitalization, and 448 had a live birth. Both symptomatic and asymptomatic women had premature births and pregnancy losses, though preterm births affected about 25 percent of symptomatic women, compared with only 8 percent of the asymptomatic women. Ten women, or 2.2 percent, some symptomatic and some not, had miscarriages or stillbirths.

(The national rate of premature births has been 10 percent in recent years, and one in 160 pregnancies result in a stillbirth, according to the C.D.C.)

The other C.D.C. report looked at 105 hospitalized pregnant women from March 1 to May 30 who were infected with the virus, most of whom had been admitted for a pregnancy-related reason or because they were in labor. Their median age was also 30. Most were asymptomatic, and more than half were Hispanic.

Among those who were hospitalized because of Covid-19, rates of obesity and gestational diabetes were higher than among those who were hospitalized for other reasons. About 30 percent of those hospitalized because of Covid required intensive care and 14 percent required a ventilator. One woman died.

Of the women who delivered their babies, 15 percent had premature births and 3 percent had stillbirths. The rates were higher than what is typically observed among pregnant women in the populations encompassed by the data set used in the study, which is called the Vaccine Safety Datalink.

“The numbers were small, and we did not control for confounders, such as prior pregnancy history — had they had pregnancy losses in the past, or other medical conditions associated with pregnancy that put them at risk?” said Dr. Lakshmi Panagiotakopoulos, a medical epidemiologist with the C.D.C. and lead author of the smaller study. “More research is needed to understand the implications.”