BACKGROUND: Size-at-birth is a crucial early endowment with long-term human-capital consequences affecting health, growth, developmental, and educational outcomes. Despite extensive research, knowledge gaps persist in this field, motivating me to investigate the literature and to harness the potential of large-scale electronic health record data to address crucial questions relevant to Palestinian refugees, the Arab World, and global discourse. THE OBJECTIVES ARE TO: 1. Conduct an umbrella review to assess the effect of various size-at-birth measures on mortality and child health, growth, and developmental outcomes, identifying associations and gaps in evidence. 2. Use data from United Nations Relief and Works Agency for Palestine Refugees in the Near East (UNRWA) to establish a birth cohort of Palestinian refugees born between 2010-2020 by linking obstetric records with child health and education records. 3. Expand the size-at-birth classification (distinguishing between gestational age (preterm, term, post-term) and size-for-gestational age (small, appropriate, and large for gestational age) to investigate the associations between being post-term, and outcomes of small-for-gestational age and infant mortality. 4. Evaluate the effects of birth size and rapid weight gain in the first year of life on childhood overweight/obesity among children aged 24 to 59 months. METHODS: In the umbrella review, I systematically searched four databases to extract systematic reviews and meta-analyses. To build the electronic record dataset, I employed deterministic linkage techniques and used decision-tree analysis to investigate linkage failures. To examine associations, I applied both logistic regression and multilevel logistic regression. Additionally, I employed structural equation models to investigate the mediating role of rapid weight gain. FINDINGS: The umbrella review identified 154 meta-analyses, underscored significant disparities in exposure definitions, and emphasized the importance of distinguishing gestational age from size-for-gestational age. In the review, I observed inconsistent associations between birth size and childhood overweight/obesity. I established a birth cohort of Palestinian refugees (n=972,743) since 2010, drawing on UNRWA data across five settings. High linkage rates were achieved. Children who died (adjusted aOR=47.0, 95% CI (44.8-49.3)) or were born to non-refugee mothers (aOR=2.7, 95% CI (2.6-2.8), were least likely to link. The study introduced a newborn-phenotype classification with nine groupings, highlighting a three times increased risk of being small for gestational age (SGA) (aRR=3.0, 95% CI: 3.0-3.1) among post-term as compared to term, and a doubling in the risk of infant mortality (aRR=2.1, 95%CI 1.7-2.6) among post-term SGA neonates compared to appropriate for gestational age (AGA) term. Analysis of size at birth found large for gestational age (LGA) newborns had nearly three times the odds of childhood overweight/obesity at 24-59 months (aOR=2.8, 95%CI (2.6-3.1)) compared to those of AGA, but no such association was observed for SGA or preterm births. Importantly, rapid weight gain during the first year of life was identified as a mediating factor in the relationship between birth size and childhood overweight/obesity. Exclusive breastfeeding is associated with a reduced odds of rapid weight gain. DISCUSSION: Overall, this research a) shows the advantages of leveraging large electronic health record datasets to provide insights into rare exposure or outcomes, b) highlights the importance of separating gestational age and size-for-gestational age, and c) emphasizes the importance of longitudinal assessment of child weight in determining the risk of overweight/obesity. This study underscores the need for targeted interventions during early postnatal phases to mitigate the risks associated with variability in size-at-birth, particularly post-term birth, and rapid weight gain.