Longitudinal Plasma Ferritin in the First Year of Life in Relation to Maternal Status, Birth Characteristics, and Breastfeeding

Background/Objectives: Iron deficiency early in life can impair infant growth and cognitive development. Here, we follow infants’ plasma ferritin levels—an indicator of iron stores—over the first year of life and relate these to birth characteristics, maternal characteristics, and infant feeding. Methods: Children and their mothers enrolled in the Swedish birth cohort NICE (ClinicalTrials.gov identifier: NCT05809479) were followed from pregnancy to twelve months postpartum. Plasma ferritin was quantified in umbilical cord blood at birth (n = 345), in venous plasma at four months after birth (mother–infant dyads, n = 133), and at twelve months of age (n = 158), using sandwich ELISA. Perinatal and postnatal growth, together with infant and maternal characteristics, were extracted from medical birth records. Breastfeeding and formula feeding were assessed using repeated monthly questionnaires during the first year. Longitudinal changes were analyzed using linear mixed-effects models, and factors associated with ferritin concentrations were examined using Spearman correlations, linear regression models, and segmented generalized additive models. Results: The ferritin concentration declined over time (birth: 267 ng/mL; four months: 146 ng/mL; twelve months: 30 ng/mL). Boys had lower ferritin levels than girls at all timepoints. Ferritin status at four and twelve months was positively associated with ferritin concentrations in cord blood and with gestational age. Breastfeeding and formula feeding were not associated with ferritin concentrations. Conclusions: Infant sex, cord ferritin concentrations, and maternal ferritin concentrations were independently associated with infant ferritin concentrations across the first year of life, whereas neither breastfeeding nor formula feeding was associated with ferritin concentrations in the present analyses. Infant sex, cord ferritin, and maternal ferritin measured four months postpartum may help identify children at risk of low iron stores, with maternal ferritin potentially offering a less intrusive alternative to repeated infant sampling. However, the clinical relevance and potential use of maternal ferritin as a proxy for infant ferritin concentrations require further investigation.