Pregnant women are four times more likely than the general population to need hospital treatment for H1N1 swine flu, data from the US suggests.
The findings suggest pregnancy does increase the risk of complications without speedy anti-viral treatment.
It also underlines the need to ensure pregnant women are made a top priority when a vaccine becomes available.
The study, by the US Centers for Disease Control and Prevention, appears in The Lancet medical journal.
Scientists studied 34 confirmed or probable cases of swine flu infection among pregnant women.
We must remember that most pregnant women who catch the disease are likely to make an uncomplicated recovery
Department of Health
They found 11 were admitted to hospital – a rate four times higher than that seen in the general population.
The women covered a broad range of ages and races and were in various stages of pregnancy. Six subsequently died after developing pneumonia.
None of the women who died had been given anti-viral drugs promptly, within the first 48 hours of symptoms occurring.
Five of these women underwent caesarean deliveries. Four of the babies have now left hospital, and the fifth, born very prematurely, is doing well. None of the babies showed any sign of swine flu infection.
The researchers accepted that doctors might be more likely to admit pregnant women to hospital than other patients – but said this was unlikely to be the only explanation for the higher rate of hospital admissions.
It may be that pregnancy weakens the immune system, making complications more likely.
Writing in The Lancet, the researchers, led by Dr Denise Jamieson, said: “On the basis of our investigation, pregnant women seem to be at increased risk for complications from pandemic H1N1 virus infection, with a higher estimated rate of hospital admission than in the general population.
“Although the decision to admit a pregnant woman is complex and might include considerations beyond simply the severity of disease, that a high proportion of influenza-related deaths in the US have been in pregnant women is concerning.”
A similar pattern was seen in the flu pandemics of 1918 and 1957, when death rates for pregnant women were higher than for non-pregnant women.
A spokesman for the Department of Health said: “This study is a reminder that pregnant women can be at increased risk from swine flu, but we must remember that most pregnant women who catch the disease are likely to make an uncomplicated recovery.
“Pregnant women should make early contact with their GP if they have flu-like symptoms.”
The spokesman said the anti-viral drug Relenza was available in an inhaled form which would not reach the foetus, and so pose no risk to it at all.
Pregnant Sharon Pentleton, from Saltcoats, Ayrshire, had to be flown to Sweden for specialist care last week after developing a rare complication following infection with swine flu.
Ms Pentleton, who is having her blood circulated through a machine, is described as “stable, but critical”.