Pregnant women, particularly those in the second half of their pregnancy, have a greatly increased risk of complications from pandemic H1N1. (CBC)

Women more than 20 weeks pregnant and suffering from H1N1 flu in Australia and New Zealand were 13 times more likely to become critically ill and be admitted to hospital than non-pregnant women, a new study suggests.

Between June 1 and Aug. 31, 2009, 209 women of child-bearing age were admitted to intensive care units with confirmed 2009 A/H1N1 influenza, or swine flu. Of these, 64 were either pregnant or had recently given birth, the researchers reported in Friday's issue of the British Medical Journal.

None of the women had been immunized against seasonal flu, despite recommendations that pregnant women should be vaccinated, the study authors noted.

Dr. Ian Seppelt of Nepean Hospital in Sydney and his co-authors from the Australian and New Zealand Intensive Care Influenza Investigators team stressed that because of the small numbers in the study, there are limits to the conclusions that can be drawn from the results.

Pregnancy and pandemics

Overall, seven women, or 11 per cent, died. Of the 60 births, four were stillbirths and three were infant deaths, a fetal loss of 12 per cent.

Although an adult mortality of 11 per cent seems low when compared with usual outcomes of respiratory failure in intensive care, it is high when compared with any other obstetric condition, the researchers said.

The study also confirmed previously reported risk factors for severe disease, including indigenous population status, presence of other conditions such as asthma, and obesity were associated with a worse outcome.

The research from Australia and New Zealand offered detailed data to enhance understanding of maternal risk and outcomes for mothers and newborns, Dr. Stephen Lapinsky of the interdepartmental division of critical care at the University of Toronto said in a journal commentary.

"Outcomes [of pregnant women with H1N1] are better than in previous pandemics, but the results are still worrying," Lapinsky concluded.

"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. This may be attributable to worldwide recommendations for pregnant women to be vaccinated against the 2009 H1N1 strain and advice to facilitate early access to antiviral treatment for pregnant women with symptoms of flu."