BACKGROUND: Like previous epidemic and pandemic diseases, 2009 pandemic influenza A (H1N1) may pose an increased risk of severe illness in pregnant women. METHODS: Statewide surveillance for patients who were hospitalized with or died from 2009 H1N1 influenza was initiated by the California Department of Public Health. We reviewed demographic and clinical data reported from April 23 through August 11, 2009, for all H1N1-infected, reproductive-age women who were hospitalized or died--nonpregnant women, pregnant women, and postpartum women (those who had delivered < or = 2 weeks previously). RESULTS: Data were reported for 94 pregnant women, 8 postpartum women, and 137 nonpregnant women of reproductive age who were hospitalized with 2009 H1N1 influenza. Rapid antigen tests were falsely negative in 38% of the patients tested (58 of 153). Most pregnant patients (89 of 94 [95%]) were in the second or third trimester, and approximately one third (32 of 93 [34%]) had established risk factors for complications from influenza other than pregnancy. As compared with early antiviral treatment (administered < or = 2 days after symptom onset) in pregnant women, later treatment was associated with admission to an intensive care unit (ICU) or death (relative risk, 4.3). In all, 18 pregnant women and 4 postpartum women (total, 22 of 102 [22%]) required intensive care, and 8 (8%) died. Six deliveries occurred in the ICU, including four emergency cesarean deliveries. The 2009 H1N1 influenza-specific maternal mortality ratio (the number of maternal deaths per 100,000 live births) was 4.3. CONCLUSIONS: 2009 H1N1 influenza can cause severe illness and death in pregnant and postpartum women; regardless of the results of rapid antigen testing, prompt evaluation and antiviral treatment of influenza-like illness should be considered in such women. The high cause-specific maternal mortality rate suggests that 2009 H1N1 influenza may increase the 2009 maternal mortality ratio in the United States. 2010 Massachusetts Medical Society