A study in Australia and New Zealand examined 64 pregnant women admitted to intensive care with swine flu during the H1N1 pandemic.
Among these cases, 68.7 per cent of women had to be put on ventilators to assist with breathing. Eleven per cent of mothers and 12 per cent of babies died after hospitalisation.
The researchers said their study was limited by the small number of pregnant women involved.
But the findings support existing data showing pregnant women are at higher risk of flu complications, they said.
The authors said none of the women in the study had been immunised against seasonal flu, despite advice that they should have been.