Postnatal Changes of Renin and Aldosterone in Term and Preterm Infants from Birth to Day 5

Background/Objectives: The renin–angiotensin–aldosterone system (RAAS) is pivotal for neonatal circulation and renal adaptation; however, postnatal changes in serum renin and aldosterone immediately after birth remain unclear. This study aimed to establish postnatal changes in these hormones at birth and over the first week of life. Methods: We retrospectively analyzed 374 neonates admitted to Kobe University Hospital between October 2020 and September 2023, with serum renin and aldosterone measured on days 0 and 5 of life. Exclusion criteria were multiple congenital anomalies, severe asphyxia, major peripartum hemorrhage, and in utero exposure to angiotensin-converting enzyme inhibitors or angiotensin II receptor blockers. Hormone levels were compared between term and preterm infants, and correlations with gestational age were assessed. Results: Serum renin concentrations were higher on day 0 than on day 5 (median 99.9 pg/mL 2.6–773.3 vs. 19.9 pg/mL 0.6–2304, p < 0.0001), and aldosterone concentrations similarly decreased (714 pg/mL 6.9–6334 vs. 551 pg/mL 0–11,930, p < 0.0001). At birth, renin and aldosterone levels did not differ significantly between groups. By day 5, both renin (32.8 pg/mL 0.6–2304 vs. 14.5 pg/mL 0.6–208) and aldosterone (689 pg/mL 4–11,930 vs. 471 pg/mL 13–4697) concentrations were significantly higher in preterm than in term neonates (p < 0.0001). Conclusions: This study describes early postnatal changes in renin and aldosterone, with higher concentrations at birth than on day 5 and persistently elevated levels in preterm infants. These findings indicate increased RAAS activity in preterm neonates and suggest a greater vulnerability to fluid, electrolyte, and blood pressure instability during early life.