Maternal metabolic determinants of accelerated infant weight gain in early life: assessing the combined effects of gestational diabetes and pre-pregnancy overweight.

Pre-pregnancy overweight and gestational diabetes mellitus (GDM) increase the risk of accelerated fetal growth, but the effect of their individual or combined exposure on infant growth trajectory remains unclear. Clarifying whether these exposures will lead to sustained or time-limited growth patterns is essential for developing tailored, time-sensitive early childhood monitoring strategies.Methods We analyzed a population-based cohort of 16,712 mother-infant pairs from Jiangsu Province (2024) using longitudinal data. Infant weight was measured at birth, 1, 3, and 6 months. Linear mixed effects models were employed to examine the independent and joint associations of pre-pregnancy overweight and GDM with infant weight-for-age z-scores (WAZ), examining how the associations changed with infant age.Results Pre-pregnancy overweight showed a strong, positive association with higher infant WAZ (β = 0.160, 95% CI: 0.130 to 0.191), with no statistically significant change over time (overweight × age interaction: β = 0.005, 95% CI: 0.000 to 0.009; P = 0.060). In contrast, GDM alone had a statistically significant but relatively small initial effect (β = 0.065, 95% CI: 0.031 to 0.100), which significantly attenuated from birth to 6 months (GDM × age interaction: β = -0.008, 95% CI: -0.013 to -0.002; P = 0.006). The combined exposure yielded the highest initial β coefficient (β = 0.217, 95% CI: 0.175 to 0.259) and the most rapid attenuation (combined × age interaction: β = -0.017, 95% CI: -0.023 to -0.010; P < 0.001). At 6 months of age, the adjusted mean WAZ remained highest in the combined exposure group. The combined effect was approximately additive in magnitude (β_combined ≈ β_OW + β_GDM), and additive interaction metrics (RERI = 0.001, 95% CI: -0.112 to 0.114; AP = 0.005, 95% CI: -0.582 to 0.592; S = 1.01, 95% CI: 0.88 to 1.15) indicated no statistical additive interaction, with confidence intervals including the null values.Conclusions Different growth patterns were found in this study: pre-pregnancy overweight showed no statistically significant attenuation over the study period, while GDM's influence was time‑limited. The combined exposure presents peak early risk that moderates over time. Clinically, infants exposed to both maternal pre-pregnancy overweight and GDM (the combined exposure group) should receive the most intensive growth monitoring during the first six months of life. These findings highlight the necessity of developing targeted monitoring protocols and implement risk stratification and personalized interventions based on maternal metabolic profiles.