Objective: To compare admissions to intensive care units (ICUs) with confirmed AH1N1v influenza in pregnancy in Australia, New Zealand and the UK. Design: National cohort studies. Setting: ICUs in Australia, New Zealand and the UK. Population: Fifty-nine women admitted to ICUs in Australia and New Zealand in June–August 2009, and 57 women admitted to ICUs in the UK in September 2009–January 2010. Methods: Comparison of cohort data. Main outcome measures: Incidence of ICU admission, comparison of characteristics and outcomes. Results: There was a significantly higher ICU admission risk in Australia and New Zealand than in the UK (risk ratio 2.59, 95% CI 1.75–3.85). Indigenous women from Australia and women with Maori/Pacific Island backgrounds from New Zealand had the highest admission risk (29.7 admissions per 10 000 maternities, 95% CI 17.9–46.3). Women admitted in Australia and New Zealand were significantly more likely to have a pre-existing medical condition (51% versus 21%, P = 0.001), but were less likely to receive antiviral treatment (80% versus 93%, P = 0.038) than women admitted in the UK. Women admitted in the UK had a longer length of hospital stay (median 21 days, range 3–128 days) than women admitted in Australia and New Zealand (median 12 days, range 3–66 days), but there were no other differences in maternal or pregnancy outcomes. Conclusions: The difference in admission risk may reflect a second phase effect from successful clinical and public health interventions, as well as differences in population characteristics between the countries. The overall severity of the AH1N1v influenza infection in pregnancy is evident, and emphasises the importance of an ongoing immunisation programme in pregnant women in both northern and southern hemispheres. ; 8 page(s)