Abstract Context Despite optimization of metabolic balance during pregnancy in type 1 diabetes (T1D), maternal–fetal complications remain higher than in the background population. Objective We examined whether there is an association between glycated hemoglobin (HbA1c) levels and these complications. Methods Retrospective study of pregnancies in 678 T1D subjects at Lille Hospital (1997-2019). The association between variations in HbA1c levels and complications was examined. The composite criterion (CC) was defined as having at least 1 of the following complications: prematurity, pre-eclampsia, large for gestational age (LGA), small for gestational age (SGA), or cesarean section. Results Among the 678 births, median preconception HbA1c was 7.2% (55 mmol/mol), 361 were LGA (56%), 29 were SGA (4.5%), and 504 were births without preterm delivery (76.1%). The CC occurred in 81.8%. Higher HbA1c during the first trimester was associated with the CC (OR 1.04; 95% CI 1.02-1.06 per 0.1% increase; P < .001). Higher HbA1c during the third trimester was associated with the CC (OR 1.07; 95% CI 1.03-1.10 per 0.1% increase; P < .001). The group defined by a first trimester Hba1c >6.5% (48 mmol/mol) and a third trimester HbA1c <6% was associated with an increased rate of the CC (OR 2.81; 95% CI 1.01-7.86) and an increased rate of LGA (OR 2.20; 95% CI 1.01- 4.78). Conclusion Elevated HbA1c is associated with maternal–fetal complications. Despite optimization of metabolic balance during the third trimester, for patients with early glycemic imbalance the risk of LGA persists.