Insulin detemir is widely used in the management of gestational diabetes mellitus (GDM) due to its favorable safety profile during pregnancy. Insulin glargine, although commonly used in non-pregnant adults, has been used more cautiously in pregnant women because of concerns about increased insulin-like growth factor-1 (IGF-1) activity and the potential fetal overgrowth. While recent studies have suggested a comparable safety profile to detemir in pregnancy, evidence directly comparing their effects on birth weight remains limited. This study aimed to compare birth weight outcomes, including macrosomia (birth weight > 4,000g) and low birth weight (LBW; birth weight < 2,500g) in offsprings of mothers with GDM treated with insulin detemir or glargine. This retrospective cohort study utilized the South Korean National Health Insurance Service (NHIS) mother-child linked database from 2010 to 2020. We used multivariable logistic regression to estimate propensity scores and applied overlap weighting to address potential confounding. We estimated risk ratios using Poisson regression and assessed mean differences in birth weight using linear regression. The study included 1,557 mother-children pairs in the glargine group and 13,065 in the detemir group. Glargine was associated with a higher risk of macrosomia (11.2% vs. 7.6%) and a lower risk of LBW (6.6 % vs. 10.2%), compared to detemir. After adjustment, glargine remained associated with an increased risk of macrosomia (RR=1.25, 95% CI: 1.06–1.48), but not with LBW (RR=0.87, 95% CI: 0.69–1.10). The weighted mean birth weight difference between the groups was 54.1g, with glargine group being heavier (p-value < 0.001). Insulin glargine use during GDM was associated with a higher risk of macrosomia and increased birth weight compared with insulin detemir. These findings support maintaining detemir as the preferred basal insulin in GDM management.