Pregnant women were more likely to get sicker from the flu than other healthy adults because they have an unusually strong immune response to influenza, a new study from the Stanford University said Monday.

The results were surprising because immune responses are thought to be weakened by pregnancy to prevent the woman's body from rejecting her fetus, according to the study published in the US journal Proceedings of the National Academy of Sciences.

"We were surprised by the overall finding," senior author Catherine Blish, assistant professor of infectious diseases at the Stanford University School of Medicine, said in a statement.

"We now understand that severe influenza in pregnancy is a hyperinflammatory disease rather than a state of immunodeficiency. This means that treatment of flu in pregnancy might have more to do with modulating the immune response than worrying about viral replication."

Blish and her team examined the reactions of immune cells taken from 21 pregnant women and 29 healthy, nonpregnant women to two flu viruses: the H1N1 strain that caused the 2009 flu pandemic and a strain of seasonal influenza, H3N2.

In the samples from the pregnant women, they found two types of white blood cells, called natural killer and T cells, had enhanced immune responses, producing more cytokines and chemokines, molecules that help attract more immune cells to the infection site to fight the virus.

"If the chemokine levels are too high, that can bring in too many immune cells," Blish said. "That's a bad thing in a lung where you need air space."

Getting the flu during pregnancy, especially pandemic strains such as those that caused the pandemics of 1918, 1957 and 2009, carries a heightened risk for pneumonia and death. Having influenza during pregnancy also quadruples a woman's risk for delivering her baby prematurely, according to the researchers.

Today, pregnant women with influenza are usually treated with drugs to slow the replication of the flu virus in their bodies. Although this is a useful treatment, the new findings suggested that it isn't the only good option, they said.

"If our finding ends up bearing out in future studies, it opens the possibility that we can develop new immune-modulating treatment approaches in the setting of severe influenza, especially in pregnant women," said lead author Alexander Kay, instructor in pediatric infectious diseases at Stanford.

The researchers planned to study whether the response of pregnant women's immune cells to other viruses would be similarly heightened.

They also urged women who are pregnant or planning a pregnancy to get their flu shots. "Flu vaccination is very important to avoid this inflammatory response we're seeing," Kay said. "But only 50 percent of pregnant women are currently vaccinated for influenza."