 Growth is traditionally used as a prognostic measure after admission to a neonatal intensive care unit NICU. Quality of neonatal intensive care is one of the factors determining the health and quality of life for those that survive, with the principal objective of increasing disability-free survival. Nowadays, there is some researches showing us that these preterm babies present a different body composition when they achieve term age compared with babies birth at term and this condition could be putting these babies in risk to metabolic syndrome early in adult age. The majority of infants born between 24 and 29 weeks of gestational age fail to achieve the mean birth weight for fetuses of the same gestational age, and many weigh below the 10th percentile at hospital discharge. The focus of this study is the growth and body composition of very low birth weight infants growing in the extrauterine environment. The central question is, when they reach the corresponding term weight, is the body composition of newborns growing outside of the uterus different from that of infants growing in the intrauterine environment? The investigators are also trying to validate 3 different methods to research body composition: DXA, air-displacement plethysmography and electric bioimpedance. 