This study explores the association between postnatal catch-up growth and obesity in childhood. It follows 848 full-term singleton infants from the Avon Longitudinal Study of Pregnancy and Childhood (ALSPAC) in the UK. The study aimed to identify predictors of postnatal catch-up growth from birth to two years and its relation to size and obesity at five years. The results showed that 30.7% of infants showed clinically significant catch-up growth, defined as a gain in standard deviation (SD) scores for weight greater than 0.67 SD scores between zero and two years. These children were lighter, shorter, and thinner at birth and had taller fathers, lower birth weights for their mothers, and were more likely to be from primiparous pregnancies. At five years, these children were heavier, taller, and had greater body mass index (BMI), percentage body fat, and waist circumference than other children. The study suggests that mechanisms that signal and regulate early postnatal catch-up growth may influence associations between small size at birth and risks for disease in adulthood. The study also examined the relationship between postnatal catch-up growth and health outcomes. It found that children who showed catch-up growth between zero and two years had greater body mass index, percentage body fat, and total fat mass at five years, indicating a higher risk of obesity and central fat distribution. The study highlights the importance of understanding the biological predisposition to catch-up growth conferred by intrauterine restraint, which may result in an acceleration of growth postnatally that overshoots the genetic trajectory. The findings suggest that early postnatal catch-up growth may contribute to the pathogenesis underlying the fetal origins hypothesis, linking small size at birth with increased risks for disease in adulthood. The study also emphasizes the need for further exploration of genetic and postnatal environmental factors that influence postnatal catch-up growth.This study explores the association between postnatal catch-up growth and obesity in childhood. It follows 848 full-term singleton infants from the Avon Longitudinal Study of Pregnancy and Childhood (ALSPAC) in the UK. The study aimed to identify predictors of postnatal catch-up growth from birth to two years and its relation to size and obesity at five years. The results showed that 30.7% of infants showed clinically significant catch-up growth, defined as a gain in standard deviation (SD) scores for weight greater than 0.67 SD scores between zero and two years. These children were lighter, shorter, and thinner at birth and had taller fathers, lower birth weights for their mothers, and were more likely to be from primiparous pregnancies. At five years, these children were heavier, taller, and had greater body mass index (BMI), percentage body fat, and waist circumference than other children. The study suggests that mechanisms that signal and regulate early postnatal catch-up growth may influence associations between small size at birth and risks for disease in adulthood. The study also examined the relationship between postnatal catch-up growth and health outcomes. It found that children who showed catch-up growth between zero and two years had greater body mass index, percentage body fat, and total fat mass at five years, indicating a higher risk of obesity and central fat distribution. The study highlights the importance of understanding the biological predisposition to catch-up growth conferred by intrauterine restraint, which may result in an acceleration of growth postnatally that overshoots the genetic trajectory. The findings suggest that early postnatal catch-up growth may contribute to the pathogenesis underlying the fetal origins hypothesis, linking small size at birth with increased risks for disease in adulthood. The study also emphasizes the need for further exploration of genetic and postnatal environmental factors that influence postnatal catch-up growth.