Maternal iron depletion trajectories during pregnancy and postpartum and their relationship with infant birthweight: A longitudinal cohort analysis

Background Maternal iron requirements increase substantially during pregnancy, and ferritin concentrations typically decline as gestation progresses. However, the physiologic significance of this decline remains uncertain, and whether reductions in maternal iron stores relate to birth outcomes is unclear. Objectives To examine associations between maternal ferritin trajectories during pregnancy and postpartum and infant anthropometric outcomes. Methods We conducted a secondary longitudinal analysis of 1,496 mother - infant pairs from the Alberta Pregnancy Outcomes and Nutrition cohort. Serum ferritin was measured longitudinally in the second and third trimesters and at three months postpartum, with limited first-trimester data available. Values below 15 g/L indicated iron deficiency. Multivariable linear regression assessed associations between inflammation-adjusted third-trimester serum ferritin and infant birthweight and length. Change in serum ferritin between the second and third trimesters ({delta} ferritin) was examined as a marker of late-gestation iron mobilization. Postpartum serum ferritin was modelled using restricted cubic splines to account for nonlinear associations with birth weight and length. Results Ferritin concentrations declined progressively across pregnancy, with 61% of women classified as iron deficient in the third trimester. Lower inflammation-adjusted third-trimester ferritin was associated with higher birthweight, corresponding to approximately 84g higher birthweight per 2.7 - fold decrease in ferritin (p < 0.001). Women experiencing the largest decline in ferritin between the second and third trimester delivered infants approximately 155 g heavier than those with minimal change (p = 0.001). Higher birthweight was associated with greater odds of postpartum iron deficiency (OR per 1 kg = 1.83; 95% CI: 1.12 - 2.99). Conclusions In this healthy cohort, maternal iron depletion in late pregnancy was associated with higher birthweight, consistent with preferential fetal iron transfer. Women delivering larger infants exhibited higher odds of iron deficiency, suggesting sustained maternal iron depletion following greater fetal iron accretion.