Prospective echocardiographic characterization of postnatal cardiac development in C57BL/6 mice: Strain analysis between birth and adulthood.

Echocardiography is critical for characterizing murine models of cardiovascular disease, yet normative data for the early postnatal period, particularly advanced functional metrics like myocardial strain, remain undefined. This developmental window features rapid cardiac maturation and is central to modeling congenital and neonatal heart disease. To establish normative, sex-specific trajectories of cardiac structure and function between birth and adulthood, we performed prospective high-frequency echocardiography in C57BL/6 mice at postnatal day 0 (P0), P8, P21, and week 8 (Wk8). Somatic growth and left ventricular structural dimensions increased progressively from P0 to Wk8. This structural maturation was accompanied by lower ejection fraction and fractional shortening, attributable to geometric remodeling. In contrast, speckle-tracking strain analysis revealed that intrinsic myocardial deformation, including global circumferential and radial strain, remained stable throughout postnatal development. Importantly, we identified sexual dimorphism emerging during late adolescence; by Wk8, males exhibited significantly greater longitudinal and radial strain magnitudes than females. This study provides the first prospective, myocardial strain-based echocardiographic characterization of murine cardiac development between birth and adulthood, identifying P21-Wk8 as a critical window for sex-specific cardiac mechanics. These normative trajectories provide a translational framework for interpreting cardiac phenotypes in preclinical models of congenital, pediatric, and developmental cardiovascular disease.