Pregnant women infected with the coronavirus are more likely to be hospitalized, admitted to an intensive care unit and put on a ventilator than are infected women who are not pregnant, according to a new government analysis.
Pregnant women are known to be particularly susceptible to other respiratory infections, but the Centers for Disease Control and Prevention has maintained from the start of the pandemic that the virus does not seem to “affect pregnant people differently than others.”
The increased risk for intensive care and mechanical ventilation worried experts. But the new study did not include one pivotal detail: whether pregnant women were hospitalized because of labor and delivery. That may have significantly inflated the numbers, so it is unclear whether the analysis reflects a true increase in risk of hospitalization.
Admission for delivery represents 25 percent of all hospitalizations in the United States, said Dr. Neel Shah, an assistant professor of obstetrics and gynecology at Harvard University. Even at earlier stages of pregnancy, doctors err on the side of being overly cautious when treating pregnant women — whether they have the coronavirus or not.
“There’s quite clearly a different threshold for hospitalizing pregnant people and nonpregnant people,” he said. “The question is whether it also reflects something about their illness, and that’s something we don’t really know.”
The results are to be published on Thursday by the C.D.C.; government researchers presented the data to a federal immunization committee on Wednesday. (The slides were posted online on Wednesday afternoon but taken down later in the day.)
The analysis, the largest of its type so far, is based on data from women with confirmed infections of the coronavirus as reported to the C.D.C. by 50 states and Washington, from Jan. 22 to June 7.
The report includes information on 8,207 pregnant women between ages 15 to 44, who were compared to 83,205 women in the same age bracket who were not pregnant.
More than 31 percent of the pregnant women were hospitalized, compared with about 6 percent of women who were not pregnant. Pregnant women were more likely to be admitted to the I.C.U. (1.5 percent versus 0.9 percent) and to require mechanical ventilation (0.5 percent versus 0.3 percent).
These proportions are small, Dr. Shah noted, and the 10-fold difference in the number of pregnant and nonpregnant women in the analysis makes it difficult to compare their risks.
In a separate analysis by Covid-Net of women hospitalized with the coronavirus, C.D.C. researchers noted that “the risk of I.C.U. and mechanical ventilation was lower among pregnant compared to nonpregnant women.” Covid-Net analyzes data from hospitalizations in the network’s surveillance area in 14 states.
Despite the ambiguities, some experts said the new data suggests at the very least that pregnant women with the coronavirus should be carefully monitored.
If many of the pregnant women were hospitalized for labor and delivery, the proportion of women who were hospitalized for only coronavirus infection and became severely ill — those advancing to the I.C.U. or ventilation — would be even higher, said Dr. Denise Jamieson, head of the Covid-19 task force at the American College of Obstetricians and Gynecologists.
“I think the bottom line is this: These findings suggest that compared to nonpregnant women, pregnant women are more likely to have severe Covid,” she said.
Pregnancy transforms the body’s biology, ramping up metabolism, blood flow, lung capacity and heart rate. It also suppresses a woman’s immune system to accommodate the fetus — a circumstance that can increase her susceptibility to respiratory illnesses like influenza.
Because of this heightened risk, scientists have been closely monitoring pregnancy outcomes in various coronavirus studies. So far, few studies have indicated a significant risk for pregnant women or for their children. Infections in newborns have been exceedingly rare.
Still, as the pandemic has progressed, prenatal care has been severely disrupted, Dr. Shah said, and women are being hospitalized for conditions that might have been caught and treated much earlier.
“Things that might have happened in an office setting are happening in a hospital triage setting,” he said.
Dr. Jamieson pointed to a recent study of pregnant women at New York City hospitals who were asymptomatic at admission. Of the 241 women who tested positive for the coronavirus in that study, 48 did not have symptoms at first but then became severely ill.
The study also found that women with more severe symptoms were more likely to give birth prematurely.
“All this information points to the importance of being vigilant when it comes to monitoring pregnant women,” Dr. Jamieson said. “They’re not at as great a risk as, for example, older people, or people with other underlying medical conditions. But they do seem to be at some increased risk.”
The data suggests that hospitals should aim to test all pregnant women for the coronavirus, regardless of symptoms, she added. The new analysis also has implications for a coronavirus vaccine, whenever one becomes available.
“How strongly are we going to counsel pregnant women about the benefits of vaccines?” Dr. Jamieson wondered.