Abstract: Objective To examine the proportion and phylogeny of overt and occult hepatitis B virus (HBV) infection (OBI) among hospitalized pregnant women.Methods We performed a prospective study which continuously recruited hospitalized pregnant women from June 1st,2012 to March 15th,2013.Demographic data were collected.Blood samples of the pregnant women were collected for the detection of HBV surface antigen (HBsAg) by using Roche^® quantitative electrochemiluminescence immunoassay and HBV S gene by using nested polymerase chain reaction and the gene was sequenced then.The differences in demographics and variation of HBV sequences between the pregnant women with overt and occult HBV infection were analyzed.Results Among the 1 130 pregnant women recruited,the proportions of overall,overt and occult (or HBsAg-negative) HBV infection were 16.6% (187/1 130),8.4% and 8.1% respectively.Totally 136 HBV strains were identified,of which 119 were classified as genotype B and available for further analysis;compared with the overt strains,the occult strains showed higher substitution rates and exhibited statistically significant minor allellic frequencies among eight nucleotide sites,leading to non-synonymous substitutions-five amino acid sites (G44E,T126A,Q129R,W156L and F200Y) in HBV S gene and four (I438L、K478N/D、N481S/D and L511F) in HBV P gene.In addition,we found three occult strains with stop codon mutation at the 35th amino acid and one occult strains with an insert mutation between the 123th and 124th amino acid.Conclusion For hospitalized pregnant women,the proportion of with occult HBV infection,routinely screened as non-HBV cases,is roughly the same as that of with overt HBV infection,which may pose a potential risk for mother-to-infant transmission.Several OBI-specific nucleotide substitutions were identified and may be linked with the occurrence of OBI.