Background: Delayed clamping of the umbilical cord incr-eases the infant’s iron endowment at birth and haemoglobin concentration at 2 months of age. We aimed to assess whether a 2-minute delay in the clamping of the umbilical cord of normal-weight, full-term infants improved iron and haematological status up to 6 months of age. Methods: 476 mother-infant pairs were recruited at a large obstetrics hospital in Mexico City, Mexico,randomly assigned to delayed clamping (2 min after delivery of the infant’s shoulders) or early clamping (around 10 s after delivery), and followed up until 6 months postpartum.Primary outcomes were infant haematological status and iron status at 6 months of age, and analysis was by intention-to-treat. This study is registered with Clinical Trials.gov, number NCT00298051. Findings: 358(75%) mother-infant pairs completed the trial. At 6 months of age, infants who had delayed clamping had significantly higher mean corpuscular volume (81.0 fL vs 79.5 fL 95%CI -2.5 to -0.6, P=0.001), ferritin (50.7 μg/L vs 34.4 μg/L95%CI -30.7 to -1.9, P=0.0002), and total body iron. The effect of delayed clamping was significantly greater for infants born to mothers with low ferritin at delivery, breastfed infants not receiving iron-fortified milk or formula, and infants born with birthweight between 2500 g and 3000 g. Acord clamping delay of 2 minutes increased 6-month iron stores by about 27-47 mg. Interpretation: Delay in cord clamping of 2 minutes could help prevent iron deficiency from developing before 6 months of age, when iron-fortified complementary foods could be introduced. Background: Delayed clamping of the umbilical cord incr-eases the infant’s iron endowment at birth and hemoglobin concentration at 2 months of age. We aimed to assess whether a 2-minute delay in the clamping of the umbilical cord of normal-weight, full- term infants improved iron and haematological status up to 6 months of age. Methods: 476 mother-infant pairs were recruited at a large obstetrics hospital in Mexico City, Mexico, randomly assigned to delayed clamping (2 min after delivery of the infant’s shoulders) or early clamping (around 10 s after delivery), and followed up until 6 months postpartum. Preliminary findings were infant haematological status and iron status at 6 months of age, and analysis was by intention-to-treat. This study is registered with Clinical Trials .gov, number NCT00298051. Findings: 358 (75%) mother-infant pairs completed the trial. At 6 months of age, infants who had delayed clamping had significantly higher corpuscular volume (81.0 fL vs 79.5 fL 95% CI -2.5 to -0.6, P = 0.001), ferritin (50.7 μg / L vs. 34.4 μg / L 95% CI -30.7 to -1.9, P = 0.0002), and total body iron. The effect of delayed clamping was significantly greater for infants born to mothers with low ferritin at delivery, breastfed infants not receiving iron-fortified milk or formula, and infants born with birthweight between 2500 g and 3000 g. Acord clamping delay of 2 minutes increased 6-month iron stores by about 27-47 mg. Interpretation : Delay in cord clamping of 2 minutes could help prevent iron deficiency from developing before 6 months of age, when iron-fortified complementary foods could be introduced.