Pregnant women are at higher risk of flu-related respiratory illnesses, which should be taken into account when weighing the risks and benefits of a flu shot during pregnancy, Canadian researchers say.

Currently, Canadian recommendations say that pregnant women should get a flu shot only if they have a pre-existing medical condition that puts them at higher risk, such as asthma or heart disease, or if they live with someone at high risk.

Pregnant women have shown higher deaths rates during influenza pandemics, but little is known about their risks during regular flu seasons.

To investigate, Dr. Linda Dodds of the departments of obstetrics and gynecology and of pediatrics at IWK Health Centre in Halifax and her colleagues looked at 13 years of data on respiratory illness among pregnant women in Nova Scotia.

Women at all stages of pregnancy showed an increased risk of flu-related respiratory illness, regardless of whether they had a high-risk medical condition, the team reports in Tuesday's issue of the Canadian Medical Association Journal.

About 2.6 per cent of pregnant women and 6.7 per cent of those with a high-risk condition had received a flu shot.

Among women without a high-risk condition, the rate of admission to hospital during the third trimester was more than twice as high during the flu season compared with the non-influenza season.

The excess hospital admissions during flu season for healthy pregnant women in their third trimester was comparable to those of Americans aged 15 to 44 who have medical conditions that put them at high risk, the researchers said.

"In conclusion, our study provides robust, population-based data showing that all pregnant women are at increased risk of influenza-associated respiratory illness," the study's authors wrote.

"Since hospital admissions because of respiratory illness during the influenza season were also increased among pregnant women without comorbidities, all pregnant women are likely to benefit from influenza vaccination."

Women and health care providers should learn about the risks of influenza during pregnancy, they said.

They suggest future studies look at the cost-effectiveness of publicly funding flu shot campaigns for all pregnant women, as well as the costs of hospital admissions, outpatient visits to doctors and broader societal costs such as missed work.