Objective: To determine daily fluctuations in weight, fluid intake, and serum sodium values in infants randomized to receive fortified human milk shortly after birth.

Methods: This is a secondary analysis of the IMPACT trial (NCT04325308) in which infants born at 28 weeks of gestation or less were randomized to receive fortified human milk either around postnatal day 3 or around postnatal day 14. The intervention was masked. Daily fluctuations in weight, fluid intake, and serum sodium values were assessed using unadjusted repeated measures mixed models. Weight measurements were converted into z-scores using the INTERGROWTH-21st growth curves. Respiratory morbidity at 36 weeks PMA or discharge was used to define bronchopulmonary dysplasia following the Jensen criteria.

Results: Data from 150 extremely preterm infants were analyzed (median gestational age: 26 weeks; mean birthweight: 750 g). Total fluid intake and enteral fluid intake did not differ between groups, but cumulative weight loss expressed in percentages (A) and declines in weight-for-age z scores (B) was more excessive in in the control group. The proportion of infants who regained birthweight by postnatal day 14 did not differ between groups. Neither the proportion of infants without BPD at 36 weeks PMA nor the proportion of infants with severe BPD differed between groups, but mild BPD was more common in the intervention group while moderate BPD was more common in the control group.

Conclusion: Early human milk fortification reduces the severity of respiratory morbidity in infants born extremely preterm without causing excessive cumulative weight loss or extending the time to regain birthweight during the first 14 days after birth.