Pregnant women are particularly at risk from swine flu and should be given priority treatment, according to the US Centers for Disease Control and Prevention (CDC).
The American equivalent of the UK Health Protection Agency reports that pregnant women with the virus are four times more likely than other sufferers to be admitted to hospital.
While it accepts that doctors may be taking a "better safe than sorry" approach, evidence, nevertheless, suggests that pregnant women face an increased risk of complications.
The report follows the case of Sharon Pentleton from Ayrshire in Scotland, who was rushed to the Karolinska University Hospital in Sweden for specialist care because no beds were available at the only UK unit able to provide the treatment, which involves artificially oxygenating the blood by circulating it through a machine.
The US scientists studied 34 cases of pregnant women with H1N1 swine flu in 13 states between 15 April and 18 May. Of those, 11-32% were admitted to hospital, four times the "normal" rate.