The Association Between Maternal ABO Blood Group and the Occurrence of Spontaneous Preterm Birth: A Retrospective Population-Based Cohort Study.

There is limited and inconsistent evidence that imply a relationship between ABO blood types and rate of preterm birth (PTB). We aim to examine the association between maternal ABO blood group and PTB rate. A retrospective-study conducted at a university teaching institution on data collected between 2013 and 2019. Women who delivered a viable neonate at ≥ 24 weeks without major malformations were included. Indicated PTBs were excluded. PTB and early PTB were defined as deliveries that occurred < 37 and < 34 weeks respectively. PTB was further divided into 3 subgroups according to etiology: membranes rupture, intact membranes, and placental abruption regardless of membranes' status. The primary outcome was spontaneous PTB rate. Of 19,301 women included, PTB and early PTB rates were 7.3% (1,418/19,301) and 2.3% (440/19,301) respectively. Rates of PTB in blood groups A, B, O, and AB, were 7.3%, 6.9%, 7.5%, and 7.5% respectively (p = 0.68). There was no significant difference according to etiology. Rates of early PTB were also comparable (p = 0.63). After adjustment for demographic and obstetric variables, blood type was associated with increased placental abruption rate among women who had early PTB (p = 0.038). Placental abruption rate was significantly higher in group A (22.5%) compared to group B (14.1%), (adjusted p = 0.04) and group O (14.0%), (adjusted p = 0.01). The rate in group AB was 17.1%, (adjusted p = 0.85). In conclusion, no association was found between a particular blood group and PTB rate. Women with group A, admitted in early PTB, had an increased risk that the underlying etiology was placental abruption.