The lack of a valid and interpretable score to track early child development over time is a primary reason for neglecting child development in policymaking. Many instruments exist, but there is no accepted method for comparing their scores across different ages, samples, and instruments. This paper aims (1) to enhance the Development Score (D-score), a unidimensional scale for early child development, to compare measurements across ages, samples, and instruments, (2) to develop a conversion key that enables the transformation of measurements obtained from existing instruments into a D-score, and (3) to investigate two new measures designed to optimize the quantification of the D-score. Study 1 gathered data from 51 sources in 32 countries among 66,075 children using 18 instruments with 2,211 items. Subject matter experts used the output of the Study-1 true score equating model to create the Global Scales for Early Development Short Form (GSED SF) and Long Form (GSED LF). Study 2 collected additional data on the GSED LF and GSED SF in three countries among 4,374 children. The Study-2 model enables the conversion of measurements into a D-score for 20 different instruments. We propose the D-score as a unifying evaluation unit to reduce fragmentation, simplify measurement, and enhance comparability.