CONTEXT: Delta-like non-canonical notch ligand 1 (DLK1) is negatively associated with bodyweight. DLK1 pathogenic variants cause central precocious puberty (CPP) and obesity, suggesting that DLK1 links the well-established association between higher body mass index and earlier pubertal onset. However, little is known about the trajectories of circulating DKL1 in healthy girls as well as in girls with precocious puberty. OBJECTIVE: To evaluate longitudinal changes in circulating DLK1 concentrations in (1) full-term, singleton healthy infant girls, (2) healthy girls during pubertal transition, and (3) girls with CPP during treatment with gonadotropin-releasing hormone agonist (GnRHa). METHODS: Three longitudinal studies of (1) healthy infant girls (n = 85), (2) healthy peripubertal girls (n = 15), and (3) girls with CPP before and after GnRHa treatment (n = 15). Body fat percentage calculated using the Slaughter equation, and serum concentrations of DLK1 using enzyme-linked immunosorbent assay. RESULTS: Serum concentration of DLK1 in healthy infant girls declined significantly through the first year of life (17.6 to 9.9 ng/mL, P = .020). DLK1 was inversely correlated with birth weight and BF%: r = -0.220, P = .044, and r = -0.503, P <
.002, respectively. DLK1 declined from 1 year prior to pubertal onset to time of first examination after pubertal onset (10.4 to 9.2 ng/mL, P = .004), as well as to time at the last pubertal evaluation (10.4 to 9.8 ng/mL, P = .006). DLK1 levels were not affected by GnRHa treatment. CONCLUSION: Circulating DLK1 levels declined steeply during infancy and were less pronounced through pubertal development. Due to considerable interindividual variation, DLK1 is not useful as a diagnostic marker of pubertal onset. Importantly, DLK1 was negatively associated with birth weight and body fat percentage.