Abstract
<title>Abstract</title> <p> <bold>Background:</bold> Low birth weight has consistently been linked to poorer cognitive outcomes, yet less is known about whether variation in birth weight within the normal range (2500–4500 g) relates to intellectual functioning in children and adolescents referred for clinical evaluation of neurodevelopmental disorders (NDDs). <bold>Objective:</bold> To examine whether birth weight within the normal range is associated with intellectual functioning and diagnostic status in a clinical sample of term-born children and adolescents with confirmed or suspected NDDs, and to evaluate whether deviations from expected birth weight are associated with diagnostic outcomes. <bold>Methods:</bold> Clinical data were obtained from participants in the Norwegian BUPgen registry with available birth weight, full-scale IQ (FSIQ), and diagnostic data. After restricting the sample to individuals born at term (37–41 weeks of gestation) with birth weight within the normal range (2500–4500 g), 248 individuals with ASD, 68 with ADHD, 7 with ID, and 26 without registered NDD diagnoses were included. Birth weight was obtained through linkage to the Medical Birth Registry of Norway. Standardized birth weight was derived using a population-based reference sample of term-born participants without NDDs from the Norwegian Mother, Father and Child Cohort Study (MoBa; n = 91,953). Linear regression models were used to examine associations between standardized birth weight and FSIQ within BUPgen. Linear models including MoBa were used to compare standardized birth weight across groups. <bold>Results:</bold> Standardized birth weight was not significantly associated with FSIQ (β=0.92, SE=0.74, p=.213). No evidence of non-linear effects (p=.625) or interactions with sex (p=.602) was observed. Mean FSIQ in the clinical cohort was also approximately 9 IQ points below the normative population mean, with significantly lower scores than the population norm observed in the total sample as well as within the ASD, ADHD, and ID groups. Linear models showed a small but significant overall effect of group on standardized birth weight (F(4, 83883) = 3.41, p = .009). Post hoc comparisons indicated that BUPgen participants without registered NDD had lower standardized birth weight than MoBa (p=.020) and ASD (p=.035), whereas no significant differences were observed between the ASD, ADHD, ID, and MoBa groups. <bold>Conclusions:</bold> Variation in standardized birth weight within the normal range was not associated with FSIQ and did not distinguish ASD, ADHD, and ID groups from the population-based MoBa sample. Although participants without registered NDD showed lower standardized birth weight, this finding should be interpreted cautiously due to the small and heterogeneous nature of this subgroup. Overall, variation in fetal growth within the normal range, as indexed by standardized birth weight, may have limited relevance for cognitive functioning and NDD diagnostic classification in clinically referred populations. </p>