The Public Health Agency has issued a warning after three pregnant women were diagnosed with swine flu in Northern Ireland.
The PHA wants all pregnant women, regardless of their pregnancy stage, to be vaccinated as soon as possible.
The agency's director of public health Dr Lorraine Doherty said that she feared pregnant women could be hardest hit by the virus.
Twenty people have died in NI as a result of the H1N1 virus.
Dr Doherty said that pregnant women, who did not get vaccinated last year, are being offered a vaccine this year.
She added: "It is particularly important that they take up this offer because pregnant women are more likely to have serious illness if they catch flu, especially swine flu."
Seventy per cent of pregnant women have received the vaccine but, according to the PHA, the numbers have "significantly" decreased over the summer months.
Dr Doherty said experience showed that pregnant women were four times more likely to develop complications, including pneumonia and heart and lung problems, and ten times more likely to be admitted to hospital.
She added that the vaccinations were completely safe for unborn babies.