Background: Fluid restriction to promote postnatal weight loss has been advocated as appropriate management of newborns with RDS. We compared changes in TBW during the first week after birth in preterm babies with and without RDS.

Subjects & interventions: 24 babies with RDS and 19 healthy babies (NRDS) were studied daily for the first week after birth; mean (range) birthweight and gestational age were: RDS 1.571 kg (0.458,3.100), 30 weeks(26,36); NRDS 1.769 kg (1.195,2.270), 32 weeks (28,34). Fluid intake, body weight and TBW (assessed using using bioelectrical impedance analysis) were recorded daily. Body solids were calculated as the difference between body weight and TBW.

Results: There was no significant difference in TBW as a percentage of body weight at birth (RDS 85.1; NRDS 85.5). Mean (SD) fluid intakes (ml/kg/day) were RDS 133 (18.3); NRDS 143 (10.1); p = 0.045). Both groups lost a similar amount of body water (RDS 10.9% (4.2); NRDS 9.9% (4.1); p = 0.45) by the end of the first week. This was unrelated to the volume of fluid administered (p = 0.41). The RDS babies lost significantly more weight by the end of the week (RDS 7.6% (4,4); NRDS 3.7 (4.0); p = 0.004). This was due to a significantly smaller gain in body solids in the RDS group (RDS 13.0%(20.0); NRDS 42.7% (40.8); p<0.01).

Conclusions: Postnatal loss of body water is similar in healthy preterms and in babies with RDS and is unrelated to the volume of fluid administered. Fluid restriction is not recommended and will only serve to limit energy intake.