Neonatal Health and Long-Term Outcomes: Twin-Based Evidence From Linked Scandinavian Registers

This study examines the long-term consequences of neonatal health using three birth measures — weight, length, and head circumference. Using linked Swedish and Norwegian population registers with follow-up into adulthood, we compare birth measures of singletons and twins, and find that different birth measures capture distinct aspects of fetal development with varying predictive validity across outcoms. Birth length emerges as the strongest predictor of health outcomes, consistent with its role as a cumulative indicator of skeletal growth throughout gestation. Birth weight retains stronger associations with cognitive and educational outcomes (grade point average [GPA], years of schooling). Head circumference shows weak and inconsistent associations across specifications. Using within-pair head circumference differences to classify twins as symmetrically versus asymmetrically growth restricted, we find that symmetric intrauterine growth restriction is associated with substantially worse long-term outcomes than asymmetric restriction. We also document significant intergenerational transmission of birth anthropometrics. Exclusive reliance on birth weight may obscure important information about early developmental insults.