INTRODUCTION Type 1 diabetes in the mother is associated with high risk of adverse neonatal outcomes. OBJECTIVES The aim of this study was to identify maternal factors associated with low arterial pH values (pH <7.10) in infants of mothers with type 1 diabetes. PATIENTS AND METHODS Data from 789 women were included in the analysis. Based on pH values in the umbilical arteries of infants, women were divided into 2 groups: those with normal pH, defined as pH of 7.1 or higher, and those with low pH, defined as pH lower than 7.1. A logistic regression analysis was used to identify the determinants of low pH in the umbilical artery, with data presented as odds ratios and 95% CIs. RESULTS Low umbilical artery pH was observed in 72 infants (9.1%). There was an association between maternal glycated hemoglobin A1c (HbA1c) levels measured before delivery and low umbilical artery pH (odds ratio [OR] 1.40; 95% CI, 1.11-1.78; P = 0.01). A similar association was found for HbA1c levels measured between 20 and 24 weeks' gestation (OR 1.29; 95% CI, 1.03-1.63; P = 0.03). There was no association between the levels of HbA1c in the first trimester or lack of preconception care and low umbilical artery pH. In logistic regression, urgent cesarean section was associated with low umbilical artery pH (OR, 1.64; 95% CI, 1.11-2.44; P = 0.01), and this association was independent of HbA1c levels measured before delivery. CONCLUSIONS Lack of sufficient glycemic control in pregnancy is the strongest predictor of neonatal acidosis in women with type 1 diabetes.