Objective To evaluate the amount of macronutrients in aggregate of human milk samples after preterm delivery during the first 2 months of lactation.

Methods Analysis of the donated single milk samples, gained by complete emptying of the whole breast at the same daytime between 24+0 and 35+6 gestational age (GA), was designed as prospective observational cohort trial. Two milk samples were analysed every postnatal week up to the discharge from the hospital, week 9 or loss of lactation. 24-Hour milk collection was not done. Analysis was performed using the MIRIS Human Milk Analyser (MIRIS AB, Uppsala, Sweden).

Results A set of 1917 human milk samples donated by 225 mothers after preterm labour was analysed. Group A (24–30 GA) contains 969 milk samples; group B (31–35 GA) contains 948 milk samples. No difference in milk composition between the groups was identified. Median of true protein content decreased from 1.6 g/dL in group A and 1.5 g/dL in group B in the first week of life, to 1.1 g/dL in both groups at the end of week 3, and then remained stable up to week 9. Content of carbohydrates and fat was stable during the whole observation, with interindividual differences.

Conclusion Human milk does not differ as a function of degree of prematurity. Protein content of preterm human milk is low and decreases during the first 3 weeks of lactation. Recommended daily protein intake cannot be achieved with routine fortification in majority of milk samples.