Depleted iron stores without anaemia early in pregnancy carries increased risk of lower birthweight even when supplemented daily with moderate iron.

Longitudinal observational study (prospective cohort) of 205 healthy, non-anaemic pregnant women aged ≥18 years, Caucasian, enrolled at 8–12 weeks gestation at Hospital Universitari Sant Joan de Reus, Catalunya, Spain, during 2005–2008; follow-up through delivery with Hb, serum ferritin and transferrin saturation measured in each trimester.

Oral iron supplementation, average 48 mg elemental iron daily, started at 17 weeks gestation (range 16–18) and continued to delivery.

Beginning pregnancy with non-depleted iron stores (serum ferritin ≥12 mg/L) is beneficial for maternal iron status and infant birthweight. In non-anaemic pregnant women receiving moderate daily iron supplementation, those with initial iron depletion (SF <12 mg/L) at 8–12 weeks had higher prevalence of iron depletion and iron deficiency during pregnancy and delivered babies weighing about 192 g less than those with non-depleted stores, after adjusting for confounders (95% CI −363.6 to −21.2; P = 0.028). The results support promoting adequate iron stores before or early in pregnancy alongside moderated daily iron supplementation to optimize birthweight and maternal iron status.

Observational design; potential residual confounding; serum ferritin can be influenced by inflammation, though transferrin saturation was used to mitigate misclassification; inflammation may still bias iron-status classification; single-hospital, predominantly Caucasian population limits generalizability; 95 of 300 recruited were excluded for incomplete data, which may introduce selection bias.