Federal health officials on Monday added pregnancy to the list of conditions that put people with Covid-19 at increased risk of developing severe illness, including a heightened risk of death.

While most pregnant women infected with the coronavirus have not become severely ill, the new caution is based on a large study that looked at tens of thousands of pregnant women who had Covid-19 symptoms.

The study found they were significantly more likely to require intensive care, to be connected to a specialized heart-lung bypass machine, and to require mechanical ventilation than nonpregnant women of the same age who had Covid symptoms. Most importantly, the pregnant women faced a 70 percent increased risk of death, when compared to nonpregnant women who were symptomatic.

The study from the Centers for Disease Control and Prevention examined the outcomes of 409,462 symptomatic women ages 15 to 44 who tested positive for the coronavirus, 23,434 of whom were pregnant.

“We are now saying pregnant women are at increased risk for severe illness. Previously we said they ‘might be’ at increased risk for severe illness,” said Sascha Ellington, a health scientist with the C.D.C., and one of the authors of the new study.

Still, Dr. Ellington emphasized that the overall risk of both complications and death was small.

“The absolute risk of these severe outcomes are low among women 15 to 44, regardless of pregnancy status, but what we do see is an increased risk associated with pregnancy,” she said.

An earlier study did not find a higher risk of dying among pregnant covid patients but the pregnant patients in the new study were 1.7 times more likely to die than nonpregnant patients. That amounted to a death rate of death of 1.5 per 1,000 cases among the symptomatic pregnant women, compared with 1.2 per 1,000 cases of symptomatic women who were not pregnant.

Even after adjustments were made for differences in age, race, ethnicity and underlying health conditions like diabetes and lung disease, the pregnant women were three times more likely than nonpregnant women to be admitted to an intensive care unit and 2.9 times more likely to receive mechanical ventilation.

Dr. Ellington emphasized the importance of taking precautions to avoid infection, saying pregnant women should not only wear masks, practice social distancing and wash their hands frequently, but also limit interactions to avoid people who may have been exposed.

“Pregnant women should be counseled about the importance of seeking prompt medical care if they have symptoms,” the authors wrote.

A smaller study, also released Monday from the C.D.C., reported that women who tested positive for the coronavirus were at increased risk for delivering their babies prematurely, finding that 12.9 percent of live births among a sample of 3,912 women were preterm births, compared with 10.2 percent in the general population. The sample was not nationally representative, but the finding echoes earlier reports that warned of a higher risk for preterm deliveries.