Pregnant women suffer more from flu than other members of the population because their immune systems are hyperactive rather than weakened, a study shows.
The discovery is unexpected since immune responses are thought to be suppressed by pregnancy to prevent a woman's body rejecting her unborn baby.
Researcher Dr Catherine Blish, of Stanford University in the US, said: "We were surprised by the overall finding. We now understand that severe influenza in pregnancy is a hyper-inflammatory 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."
The researchers took immune cells from 21 pregnant and 29 healthy, non-pregnant women and exposed them to different flu viruses in the laboratory.
Cells taken from women six weeks after they had given birth were also tested.
Pregnancy boosted the immune response to swine flu, the H1N1 strain that caused a pandemic in 2009, by affecting two types of white blood cell, natural killer (NK) and T-cells.
Compared with those from non-pregnant women, both cell types produced larger amounts of signalling molecules that attract other immune cells to infection sites.
This could lead to lungs becoming clogged up by an influx of immune cells, Dr Blish said.
Catching flu when pregnant, especially pandemic strains, is known to heighten the potentially fatal risk of pneumonia.
Swine flu and the seasonal flu strain H3N2 also caused NK and T-cells to be activated in a greater variety of ways in pregnant women, the researchers reported in the journal Proceedings of the National Academy of Sciences.
Lead author Dr Alexander Kay, also from Stanford University, 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."
He hoped the research would remind 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," Dr Kay said. "But only 50 per cent of pregnant women are currently vaccinated for influenza."