Alicia M. Siston of the Centers for Disease Control and Prevention in Atlanta and colleagues analyzed data on influenza cases of pregnant women reported from April through December of last year.

From April to August of last year, 788 pregnant women with influenza A H1N1 were reported. Among these pregnant women, 30 died, 509 were hospitalized and 115 were admitted to an intensive care unit.

The study, published in the Journal of the American Medical Association, found pregnant women who were treated more than four days after symptoms appeared were six times more likely to be admitted to an intensive care unit than those treated within two days after symptoms onset.

"Pregnant women represent approximately 1 percent of the U.S. population, yet they accounted for 5 percent of U.S. deaths from 2009 influenza H1Nl reported to the CDC," the study authors said in a statement.

"The data are consistent with previous studies that demonstrate that pregnant women with influenza are at increased risk of serious illness and death."