Pregnancy itself is considered a risk factor for severe disease [3] but the biological basis for this has not been established. Pregnancy-associated changes in immune function, hormone levels, cardiopulmonary stress and difficulties in treatment for respiratory disease are often cited as important factors [52] . The presence of other risk factors may increase the risk of severe disease in a pregnant woman (Figure 3) . The presence of a known co-morbidity in pregnant women with severe 2009 H1N1 disease can vary greatly and has been documented as 16% to 56% [37, [46] [47] [48] [49] [50] 53] . There is no one or cluster of co-morbidities associated with increased disease severity in pregnant women. Data indicate that when no additional co-morbidities were present, pregnant women still had a seven-to tenfold higher rate of severe disease when compared to age-matched, nonpregnant women [49, 50] .