Pregnant women may be far more at risk from swine flu than thought, according to a survey published on Friday that was carried out in Australia and New Zealand.
An investigation carried out among American women between April and May last year, in the first month of the H1N1 virus' outbreak, suggested pregnant women were four times likelier to develop severe illness requiring hospitalisation compared with non-pregnant counterparts.
But the new paper, published online by the British Medical Journal (www.bmj.com), found in a small sample that the risk of critical illness was between seven and 13 times as much.
From June 1 to August 31 last year, 209 women of child-bearing age were admitted to an intensive care unit (ICU) in Australia and New Zealand with confirmed swine flu, 64 of whom were either pregnant or had recently birth.
The fact that almost one in three of the admissions were women who were pregnant or post-partum was significant, says the paper.
When extrapolated to the child-bearing population of both countries, a pregnant or post-partum woman with H1N1 was more than seven times likelier to be admitted to an ICU compared to a non-pregnant counterpart who also had flu.
In the case of flu-infected women who were more than 20 weeks' pregnant, ICU admission was 13 times higher than a non-pregnant, infected counterpart.
The study also showed high rates of ventilator and oxygen use to help the pregnant women breathe.
Seven of the women died, four babies were stillborn and three died in infancy.
Of the surviving births, 22 were pre-term and 32 needed treatment at a neonatal intensive care unit.
None of the women had been vaccinated against seasonal flu, despite recommendations that pregnant women be immunised.
In an editorial, Stephen Lapinsky, a professor of critical-care medicine at the University of Toronto in Canada, said mortality from H1N1 during pregnancy seemed high, although not as bad as in previous flu pandemics.
In the 1918-9 outbreak of so-called Spanish flu, which slew tens of millions of people, maternal mortality was reported in one study to be as high as 27 percent, while in the 1957-8 "Asian" flu pandemic, which may have killed two million, half of the women of child-bearing age who died were pregnant.
"Despite evidence of an increase in maternal mortality after infection with H1N1, in the later phases of the pandemic its effect on pregnant women has been less than was initially anticipated," said Lapinsky.
This could be explained by vaccination advice and early use of antiviral drugs to women with flu symptoms of flu, he suggested.
H1N1 caused at least 16,713 deaths as of March 7, according to a March 12 posting on the World Health Organisation website (www.who.int).