Objective: The ABO blood types are associated with cancers, cardiovascular disease, and Type 2 diabetes in adult males and females. Associations of ABO blood type and adverse pregnancy outcomes have not been extensively studied. The purpose of this study was to investigate the relationship with ABO blood groups and risk of adverse pregnancy outcomes contributing to premature birth.Study design: Data on ABO phenotypes and pregnancy outcomes were collected from the medical records of 1,462 premature infants (22-34 weeks). Adverse pregnancy outcomes that were studied in relation to maternal blood type included gestational hypertension (GHTN), preeclampsia (PREE), chorioamnionitis (CA), preterm premature rupture of membranes (PPROM), and intrauterine growth restriction (IUGR)Results: 1,462 charts of mothers with premature infants (22-34 weeks) were studied measuring the relative risk by using standardized statistical software (SPSS).Our study group had 46 mothers with GHTN, 405 with PREE, 282 with CA, 504 with PPROM, and 94 with IUGR. A+ Caucasian mothers had a 28% increased risk of developing preeclampsia (RR= 1.28 (1.09-1.52); 95% CI= .003). B mothers are ata 46% decreased risk of developing chorioamnionitis versus all other blood groups (RR= 0.54, 95% CI= 0.36-0.81; P= .003). Conversely, O+ Caucasian mothers were 2.53 times more likely to develop chorioamnionitis compared to all other blood types (RR=2.53, 95% CI= 1.09-5.88; P= .031).Conclusions: Maternal ABO phenotype significantly influences the incidence of preeclampsia and chorioamnionitis. Pregnancy is a unique antigen-antibody phenomenon with the fetus serving as an antigen to the mother. We postulate that blood group antigen expression at the endothelial level may influence maternal disease states.