Maternal-Newborn ABO Blood Groups and Risk of Bacterial Infection in Newborns.

Importance: Newborn immunity largely relies on maternal-fetal transfer of antibodies in utero. Incongruency in ABO blood groups between a mother and newborn may be associated with protection against serious infections, but data specific to newborn bacterial infections are lacking. Objective: To ascertain the association between maternal-newborn ABO blood group incongruence and lower risk of bacterial infection in newborns. Design, Setting, and Participants: This cohort study used linked patient-level datasets for all singleton live births between January 1, 2014, and December 31, 2020, in hospitals and health centers in Ontario, Canada. The cohort comprised maternal-newborn pairs with known ABO blood groups. Data analysis was conducted between February and May 2024. Exposure: Maternal-newborn ABO blood group incongruence vs congruence. Main Outcomes and Measures: The primary outcome was a bacterial infection arising in newborns within 30 days of birth. Bacterial infection was cultured from either blood, cerebrospinal fluid, urine, or lung specimen. Secondary outcomes were a bacterial infection with 7 days and 90 days of birth. Modified Poisson regression generated adjusted relative risks (ARRs) and 95% CIs, adjusted for neonatal sex and preterm birth. Results: A total of 138 207 maternal-newborn pairs (maternal mean [SD] age, 31.8 [5.1] years among those with ABO blood group incongruency and 31.5 [5.1] years among those with ABO blood group congruency; newborn mean [SD] gestational age, 38.5 [2.3] weeks among those with incongruency and 38.4 [2.5] weeks among those with congruency; 19 475 males [51.3%] with incongruency and 52 041 males [51.9%] with congruency) were analyzed. Of these pairs, 37 953 (27.5%) had ABO blood group incongruency and 100 254 (72.5%) had ABO blood group congruency. Within 30 days of birth, 328 (8.6 per 1000) newborns in the incongruent group and 1029 (10.3 per 1000) newborns in the congruent group experienced a bacterial infection, corresponding to an ARR of 0.91 (95% CI, 0.81-1.03). The ARRs for bacterial infection within 7 days and 90 days of birth were 0.89 (95% CI, 0.73-1.09) and 0.86 (95% CI, 0.78-0.94), respectively. Conclusions and Relevance: This cohort study found no association between maternal-newborn ABO blood group incongruence and risk of bacterial infection in newborns within 30 and 7 days of birth. However, incongruence was associated with a decreased risk of bacterial infection within 90 days of birth.