Fluid Output in the First Week May Shape Severe Lung Disease in Preterm Infants

The first days of life are a physiological tightrope for infants born very preterm, and few clinical variables are managed as intensively, and as anxiously, as fluids. Neonatologists weigh every milliliter delivered through intravenous lines against every milliliter lost through urine, insensible evaporation, and other routes, guided by the long-standing conviction that keeping cumulative fluid balance within a narrow range protects fragile developing lungs. A new retrospective cohort study from Shanghai Children’s Medical Center now complicates that picture in a striking way. The research, published in BMC Pediatrics, followed 211 infants born before 32 weeks of gestation and found that the total volume of fluid these babies lost during the first postnatal week was independently associated with whether they went on to develop moderate-to-severe bronchopulmonary dysplasia, one of the most feared complications of extreme prematurity. Fluid intake, by contrast, showed no significant independent association once the two variables were disentangled statistically.

Bronchopulmonary dysplasia, or BPD, is a chronic lung disease defined clinically by the continued need for respiratory support or supplemental oxygen at 36 weeks of postmenstrual age. It affects a substantial fraction of very preterm survivors and carries lifelong consequences, including increased risks of asthma-like symptoms, rehospitalization, impaired growth, and neurodevelopmental difficulties. The disorder arises from an intricate interplay of prematurity itself, inflammation, mechanical ventilation, oxygen toxicity, infection, and disturbed fluid homeostasis. Because the preterm lung must transition from a fluid-filled fetal state to an air-filled neonatal one within minutes to days, the way water moves into and out of the body during that transition has long been suspected to matter. Excess extracellular water can flood the pulmonary interstitium, worsen compliance, prolong the need for ventilation, and thereby amplify lung injury. This biological logic underpins the standard practice of restricting fluid intake and watching for a postnatal weight loss of roughly five to fifteen percent in the first week as a sign that the infant is clearing fluid appropriately.

The new study set out to characterize the longitudinal trajectories of fluid output, fluid balance, and related indicators across the first seven days of life, and then to test how those trajectories relate to moderate-to-severe BPD, classified according to the 2018 NICHD criteria. The investigators assembled a single-center retrospective cohort of very preterm infants who survived to 36 weeks postmenstrual age, and they applied generalized linear mixed models, a statistical framework well suited to repeated daily measurements nested within individual patients. This approach allowed them to compare day-by-day curves of fluid intake, output, balance, intake-to-output ratio, and weight loss between infants who later developed severe lung disease and those who did not, while adjusting for confounding factors. Their primary exposure was the cumulative fluid output over postnatal days three through seven, a window chosen because the earliest days are dominated by physiological transitions and measurement noise, whereas days three to seven better reflect sustained fluid handling.

Among the 211 infants included, 77, or 36.5 percent, developed moderate-to-severe BPD, a rate consistent with the substantial burden of disease in this gestational age range. When the daily trajectories were compared, one signal stood out: fluid balance over the first week behaved differently in the two groups, with a statistically significant BPD-by-time interaction for daily balance. Daily output, intake, intake-to-output ratio, and percentage weight loss did not differ significantly day by day between the groups. Yet when the researchers turned to cumulative measures across days three to seven, the pattern became sharper and, in one respect, inverted expectations. Every 10 milliliter per kilogram increase in cumulative fluid output over that five-day span was independently associated with higher odds of moderate-to-severe BPD, with an adjusted odds ratio of 1.073 and a 95 percent confidence interval of 1.031 to 1.115.

The crucial methodological move in the study was to separate output from intake. In observational neonatal data, output and intake are tightly coupled, because clinicians often adjust prescribed fluids in response to what the infant is losing, and because balance is by definition the arithmetic difference between the two. A naive analysis of net balance alone can therefore conflate the effects of giving too much fluid with the effects of losing fluid. The authors addressed this by constructing a joint model containing both cumulative output and cumulative intake over days three to seven. In that model, output retained a robust independent association with BPD, while cumulative intake was not statistically significant, with an adjusted odds ratio of 0.961 and a confidence interval of 0.924 to 0.999 that just crossed the null. A reference model using cumulative balance alone showed a reciprocal protective-sounding trend, with an adjusted odds ratio of 0.942, meaning a more positive balance appeared associated with lower odds of severe disease, a finding that is the mirror image of the output result and underscores how strongly the choice of exposure metric shapes conclusions.

That inversion is what makes the study provocative. Conventional neonatal teaching emphasizes cumulative net balance as the key target: a positive balance, meaning more fluid retained than excreted, is traditionally viewed as a risk factor for BPD because it implies pulmonary edema. The new data instead suggest that infants who went on to develop severe lung disease were, if anything, those with higher fluid losses in the first week, even after accounting for how much fluid they received. Several biological interpretations are possible, and the authors are careful to frame their findings as hypothesis generating rather than practice changing. High output could be a marker rather than a cause: infants who are sicker from the start, exposed to more inflammation or receiving nephrotoxic or diuretic medications, may both lose more fluid and be more likely to develop BPD for reasons unrelated to water handling. Alternatively, excessive fluid losses could reflect immature renal concentrating ability, and the resulting dehydration, electrolyte disturbance, and reduced circulating volume might impair perfusion and recovery of the developing lung.

There is also a plausible measurement story. Insensible water losses through the skin and respiratory tract of extremely preterm infants are enormous and difficult to quantify precisely, particularly under radiant warmers or phototherapy, and recorded output in retrospective chart data may miss stool losses or capture errors in weighing diapers. The daily trajectories analysis supports this caution: daily balance, a composite that integrates unmeasured losses implicitly, did show a significant group-by-time difference, while directly recorded daily output did not. Cumulative indices over days three to seven may smooth random error and reveal signal that single-day comparisons miss, but they also amplify any systematic bias in how fluids are charted. The single-center design, while ensuring consistent local practice, limits generalizability to centers with different fluid protocols, humidification strategies, or ventilation practices, and the retrospective nature means confounding by indication can never be fully excluded, even with statistical adjustment.

Statistically, the adjusted odds ratio of roughly 1.07 per 10 milliliters per kilogram is modest in magnitude but clinically meaningful when applied to the large fluid volumes handled over five days: a cumulative output difference of 100 milliliters per kilogram would translate into nearly a doubling of the odds in this model. Still, the confidence interval for intake came tantalizingly close to significance, and the reciprocal balance result suggests that the underlying associations are entangled in ways that only prospective, protocolized data collection can resolve. The study was retrospectively registered in the Chinese Clinical Trial Registry on 10 January 2025, and it was approved by the Institutional Review Board of Shanghai Children’s Medical Center with the consent requirement waived for the use of de-identified retrospective data. The authors declare no competing interests, and the work received no external funding.

For clinicians, the immediate takeaway is not to abandon careful fluid restriction but to pay closer attention to the output side of the ledger. Tracking cumulative output during days three to seven may offer an early window into which infants are at heightened risk of severe BPD, potentially prompting intensified respiratory care, closer renal monitoring, or earlier engagement of families in discussions about long-term follow-up. For researchers, the study identifies a clear agenda: prospective cohorts that simultaneously measure insensible losses, renal biomarkers, biompedance-based estimates of extracellular water, and echocardiographic markers such as hemodynamically significant patent ductus arteriosus, to determine whether high output is a cause, a consequence, or a correlated signature of lung vulnerability. Until such studies arrive, the authors’ message is measured but pointed. The neonatal fluid conversation has been dominated by how much fluid goes in; these findings insist that how much comes out deserves equal scientific scrutiny, and that the humble daily fluid chart, long treated as bookkeeping, may contain underexploited clues to one of prematurity’s most stubborn complications.

Subject of Research: Association of first-week fluid output trajectories with moderate-to-severe bronchopulmonary dysplasia in very preterm infants

Article Title: Association between fluid trajectories in the first postnatal week and moderate-to-severe bronchopulmonary dysplasia in very preterm infants: a retrospective cohort study

Article References: Wang, S., Chen, X., Bei, F., & Bu, J. (2026). Association between fluid trajectories in the first postnatal week and moderate-to-severe bronchopulmonary dysplasia in very preterm infants: a retrospective cohort study. BMC Pediatrics. https://doi.org/10.1186/s12887-026-07728-z

Image Credits: AI Generated

DOI: 10.1186/s12887-026-07728-z

Keywords: bronchopulmonary dysplasia, fluid output, fluid balance, very preterm infant, neonatology, retrospective cohort study, fluid management, prematurity, chronic lung disease, postnatal fluid trajectories, Association, between