Pregnant women are more vulnerable to flu
Pregnant women are worse hit by flu than other members of the population because their immune systems are hyperactive rather than weakened, a study has shown.
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, from 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, said Dr Blish.
Lead author Dr Alexander Kay hoped the research would remind women to get their flu shots.