L-Citrulline in Maternal–Fetal and Neonatal Health: Metabolic Mechanisms and Emerging Therapeutic Applications

L-citrulline is a non-protein amino acid with critical roles in perinatal and neonatal physiology through the intestinal–renal arginine–citrulline axis and nitric oxide (NO) production. During pregnancy, L-citrulline supports placental angiogenesis, vascular adaptation, and fetal growth through augmentation of arginine availability and endothelial NO production. In neonates, particularly preterm infants, developmental immaturity of citrulline and arginine synthesis contributes to hypoargininemia and may increase susceptibility to necrotizing enterocolitis, bronchopulmonary dysplasia with pulmonary hypertension, sepsis, and impaired intestinal function. Although L-citrulline has emerged as a promising modulator of NO bioavailability, prior reviews have largely focused on either adult cardiovascular disease or isolated neonatal applications, with limited integration of its mechanistic and translational relevance across the perinatal and neonatal continuum. Collectively, current evidence supports L-citrulline as a promising translational target in maternal–fetal and neonatal medicine because of its central role in vascular, inflammatory, and metabolic regulation. However, adequately powered clinical trials are needed to define optimal dosing, timing, patient selection, and long-term outcomes before routine clinical implementation can be recommended. This review provides a comprehensive evaluation of L-citrulline metabolism and its therapeutic potential from pregnancy through neonatal life, with emphasis on the intestinal–renal arginine–citrulline axis, endothelial function, and NO-mediated vascular regulation. We specifically examine the role of citrulline in key pathophysiologic mechanisms underlying maternal and neonatal disease, including endothelial dysfunction, impaired NO bioavailability, inflammation, oxidative stress, and abnormal placental vascular remodeling.