Introduction Case series suggest pregnant women are at increased risk of contracting H1N1 and experiencing complications. Published studies to date have investigated symptomatic patients or ascertained serology cross-sectionally. Such studies do not allow accurate quantification of incidence and neglect mild disease when estimating complication rates. We investigated H1N1 incidence and rate of complications in unvaccinated women in Scotland during the winter 2009/10 pandemic.

Method We recruited 417 unvaccinated pregnant women who attended hospitals in NHS Lothian in Dec 2009-April 2010. Participants completed a research nurse-administered questionnaire, had venous blood taken and clinical outcomes were extracted from hospital records. Booking blood samples (collected routinely at 10–14 weeks gestation) were retrieved for each participant to allow testing of paired blood samples using the microneutralisation assay. Evidence of infection during pregnancy was defined as a 10-fold increased in H1N1 antibody titre between booking and delivery.

Results Seroconversion between booking and delivery occurred in 10.5% (95% CI 7.1% to 13.9%) with 19 of 32 unaware of acquiring infection. Self-reporting flu symptoms and asthma (but no other chronic conditions) were statistically significant predictors of infection. No significant differences were found in rates of maternal or neonatal hospital admission, critical care admission, birth weight or adverse events between those infected and uninfected.

Conclusion In Scotland where estimated coverage of H1N1 vaccination in pregnancy was 47.6%, relatively few unvaccinated pregnant women experienced H1N1 infection with many infected experiencing minimal symptoms. No increased risk of adverse events was detected but we have low power for this analysis.