US study shows how pregnancy can increase risk of flu complications

Medical science has long assumed that the higher risk of hospitalisation and death due to the flu among pregnant women was down to the suppressed immunity that keeps the body from rejecting the fetus.

But new research suggests the opposite: that the immune system goes into hyperdrive in pregnant flu patients, and this is what makes the women ill.

The results, published online on Monday in the journal Proceedings of the National Academy of Sciences, could lead to new ways to combat influenza infection among pregnant women.

It is true that women's immune systems are strongly suppressed, overall, during pregnancy. But researchers suspected this alone could not explain pregnant women's increased vulnerability to influenza. It seemed to contradict chemistry and common sense.

Recent studies had shown there was a complicated mix of chemicals promoting and combating inflammation among the white blood cells of pregnant women infected with the virus. And pregnant women with the flu frequently suffer from severe congestion, a symptom that is typical of an inflammatory response on overdrive.

"Nobody bothered to look at the direct viral response before," said Dr Catherine Blish, an immunologist at Stanford University School of Medicine and principal investigator.