Introduction Pregnant women are at increased risk of contracting H1N1 and developing complications. However, the incidence of asymptomatic or mild H1N1 infection during pregnancy is not known. We investigated H1N1 incidence and rate of complications in unvaccinated women who were pregnant during the 2009/2010 winter pandemic.

Method 417 unvaccinated pregnant women in Lothian were recruited at delivery between December 2009–April 2010. Women completed a questionnaire and had a blood sample taken. Clinical outcomes were recorded. Antenatal H1N1 infection was defined as a positive H1N1 swab during pregnancy, or a 10-fold increase in antibody titre between booking and delivery (using the microneutralisation assay).

Results 10.5% (95% CI 7.1% to 13.9%) of women had H1N1 infection during pregnancy. There was no difference in maternal age, body mass index and smoking status at booking or gestation at peak of the pandemic between those infected and uninfected (all p>0.05). Self-reporting flu symptoms and asthma were the only predictors of H1N1 infection (0.03 and 0.001, respectively). No increased risk of adverse clinical outcomes was detected in infected compared to uninfected women, but we have low power for this analysis.

Conclusion During the 2009/2010 winter H1N1 pandemic, relatively few unvaccinated pregnant women contracted H1N1 infection, with most experiencing minimal symptoms. However, given that only 47% of pregnant women were vaccinated, many unvaccinated women were still susceptible to H1N1 infection. These are a priority group for vaccination and a clearly targeted message needs to be given to pregnant women to promote vaccination especially in a pandemic.