Based primarily on U.S. figures from the first few months of the global epidemic, which began last April, CDC officials believe that though pregnant women account for just 1 percent of the population, they have at times accounted for as many as 5 percent of swine flu deaths.

The analysis found that only one of the U.S. women who died was treated with flu medicine like Tamiflu within the first two days of symptoms; just four of those who died got treatment within the first four days.

"Early treatment really makes a difference," said Dr. Sonja Rasmussen of the CDC, one of the study's authors.

The report appears in Wednesday's Journal of the American Medical Association.

The study only looked at deaths in 2009, the year the virus first emerged. Deaths that occurred this year were not part of the analysis, and officials don't now how many pregnant women died this year. The first American with swine flu to die was a Texas woman who was days away from giving birth. Judy Trunnell, 33, died after slipping into a coma. Her daughter was delivered by cesarean section and was healthy.

For all of 2009, the researchers tallied 280 women who were treated in intensive care, including the 56 who died.

Like Trunnell, most of the deaths were women who were late in their pregnancy. Among the 30 pregnant women who died in the first four months of the pandemic, 60 percent were in their third trimester.

Also like Trunnell, many of the women had other health problems that made them more susceptible to severe complications: asthma was the most common problem - 44 percent of the pregnant women who died in the first months had asthma; about 39 percent were obese.

Even healthy pregnant women are unusually susceptible to seasonal flu, Rasmussen said. Pregnancy changes the immune system so the body adapts to the fetus being carried, but makes it more vulnerable to flu. Pregnant women also have a faster heartbeat and more trouble breathing because of the fetus.

Most of the report's findings echo what smaller studies found earlier, but the results about the effectiveness of flu medicines were striking. So too was the authors' conclusion that a possible reason flu drugs weren't given to some women right away - some doctors may have relied too much on rapid tests that falsely signal no flu virus as often as 70 percent of the time.

"If that's true, the test really did more damage than helped" by causing doctors to delay giving antivirals to pregnant women, said Dr. Richard Wenzel, and infectious diseases specialist at Virginia Commonwealth University.