They are also more likely to die of swine flu or even seasonal flu, and yet only 15 percent of pregnant women in the United States follow the advice of the Centers for Disease Control and Prevention (CDC) and get an annual flu shot.

Data collected and analyzed by the CDC for the study showed there were 34 confirmed cases of (A)H1N1 influenza among pregnant women in the United States between mid-April and mid-May, the first month of the outbreak that has since been declared a pandemic by the World Health Organization.

Eleven of the pregnant women, or about a third, were admitted to hospital. The hospitalization rate for members of the general public infected with swine flu was around eight percent, or a quarter of the rate among pregnant women, the study showed.

One of the women sickened in that first month died, but five more pregnant women died of swine flu in the United States during the following month of the outbreak, the report said.

All of them had developed pneumonia and acute respiratory distress syndrome requiring mechanical ventilation, it said.

"We know that in seasonal influenza as well as in pandemic influenza situations that pregnant women have an increased risk of severe disease and of dying," the CDC's Denise Jamieson, lead author of the study which will be published in the August 8 issue of the Lancet, told AFP.

The increased risk is likely due to the changes that take place in a woman's body during pregnancy, she said.

"There are mechanical and hormonal changes in pregnancy, there are changes in the cardiovascular and respiratory systems, there are immunologic changes," Jamieson explained.

"Lung capacity decreases because as the uterus grows it moves the diaphragm up and there's basically less room for the lungs. All these changes make pregnant women more susceptible to and more severely affected by certain viruses, including influenza."

The six pregnant women who died sought medical care within one to seven days of showing flu-like symptoms. All were given anti-virals which have been shown to be effective against the new strain of swine flu, but none received the medication until at least six days after the onset of symptoms.

Anti-virals oseltamivir and zanamivir, which have been shown to be effective against the (A)H1N1 virus, are most successful in reducing the severity of flu if started within 48 hours of the onset of illness.

The CDC began closely monitoring (A)H1N1 infections in pregnant women in May after a previously healthy, 33-year-old woman in her 35th week of pregnancy became the second person in the United States to die of swine flu.

The United States has confirmed 302 swine flu deaths as of July 24, the highest death toll in the world.

Pharmaceutical companies are racing to develop a vaccine against the new strain of swine flu, and the CDC's Advisory Committee on Immunization Practices (ACIP) is due to meet Wednesday to establish recommendations on whom to vaccinate against (A)H1N1 in the United States when flu season returns in the autumn.

"Since there's not yet an H1N1 vaccine, there are not yet recommendations about pregnant women. But we do recommend that pregnant women be vaccinated against seasonal influenza," said Jamieson.

Despite the CDC recommending since 2004 that expectant US mothers be inoculated against seasonal flu, less than 15 percent take up the recommendation, partly out of fear of the possible side-effects on their unborn baby, she said.

The babies of five of the six pregnant women who died during the study period were delivered by Caesarean section.

None had any evidence of influenza infection and all but one, who was born 13 weeks before term, have been discharged in good health, the study said.

The baby born at 27 weeks was still in hospital and doing well.

The sixth woman was only 11 weeks pregnant and her baby died when she did, the study said.

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