BACKGROUND: Pregnant women were at increased risk for serious outcomes of 2009 
pandemic influenza A virus subtype H1N1 (influenza A[H1N1]pdm09) infection, but 
little is known about the overall impact of the pandemic on neonatal and 
maternal outcomes.
METHODS: We identified live births that occurred from 1 July 2008 through 31 May 
2010 in 5 Kaiser Permanente regions. Pregnant women were considered to have 
influenza if they had a positive result of a laboratory test for influenza virus 
or if they received a diagnosis of influenza during a period in which seasonal 
influenza virus or A(H1N1)pdm09 was the predominant circulating virus.
RESULTS: There were 111 158 births from 109 015 pregnancies involving 107 889 
mothers; 368 pregnant women (0.3%) received a diagnosis of influenza due to 
seasonal virus, and 959 (0.9%) received a diagnosis of influenza due to 
A(H1N1)pdm09; 107 688 did not receive an influenza diagnosis. Pregnant women 
with influenza due to A(H1N1)pdm09 were more likely than women with seasonal 
influenza infection to be hospitalized within 30 days of the diagnosis (27% vs 
12%; odds ratio [OR], 2.84 [95% confidence interval {CI}, 2.01-4.02]). Pregnant 
women with A(H1N1)pdm09 who started antiviral treatment ≥2 days after the 
diagnosis were significantly more likely to be hospitalized than those who 
started antiviral treatment <2 days after diagnosis (OR, 3.43 [95% CI, 
1.55-7.56]). Mothers with seasonal influenza virus infection had an increased 
risk for having a small-for-gestational-age infant (OR, 1.59 [95% CI, 
1.15-2.20]).
CONCLUSIONS: In this large, geographically diverse population, A(H1N1)pdm09 
infection increased the risk for hospitalization during pregnancy. Late 
initiation of antiviral treatment was also associated with an increased risk for 
hospitalization.