**2. Methods**

There is a specific program in our region (Comunidad Foral de Navarra), fostered by the Regional Health Service, that intends to promote healthy lifestyle and monitors child growth and development. By means of periodic consultation (usually at birth and during the first year, and ages 0.5, 1, 2, 3, 4, 6, 8, 10, 12 and 14 years), the program accomplish the registration of anthropometric measurements (weight and height) and saves different data in the clinical records.

These children, who represent a cohort of VLBW infants (<1500 g), have been evaluated by a pediatrician and/or pediatric nurse by the use of the different facilities available (our public Health Service guarantees universal accessibility in distance and personal assistance). The different consultations were programmed at birth and ages 0.5, 1, 2, 3, 4, 6, 8, 10, 12 and 14 years. A brief medical history, basic physical exam and the anthropometric measurements (weight and height) were recorded. The only requirements to be included were a Caucasian origin from Spanish parents, and the birth place (the Neonatal Unit of the Navarra Hospital Complex in Pamplona, Spain, which is the reference Hospital) in the period January, 2001-December, 2005.

*Characteristics of Catch-Up Growth in Very Low Birth Weight Infants (<1500 g) DOI: http://dx.doi.org/10.5772/intechopen.96933*

Body measurements (weight and height) were taken during physical exam in underwear and barefoot. We used an Año-Sayol scale (reading interval 0 to 120 kg and a precision of 100 g) for the measurement of weight and a Holtain wall stadiometer (reading interval 60 to 210 cm, precision 0.1 cm) for the measurement of height.

We collected a sample of 217 births of babies who met the criteria of VLBW, 47 of whom were excluded due to different reasons: perinatal mortality in 20 (9.2%), the finding of severe malformations or chromosomopathies in 6 (2.8%), severe neurosensory disability and motor sequelae in 5 (2.3%), ethnic origin in 8 (3.7%) and other reasons (geographical distance to hospital and difficulties for transportation, absence of continuity in the evaluations of the pediatric health screening, etc.) in 8 cases (3.7%).

The children (VLBW) in this cohort were divided in two subgroups: newborn infants who were appropriate for gestational age (AGA subgroup) and newborn infants who were small for gestational age (SGA subgroup). The difference between both groups was that birth weight and/or length were higher or equal/lower than two standard deviations below the average of a reference population for gestational age and sex, respectively. We used the growth reference charts for newborns from the anthropometric growth patterns of preterm from Carrascosa et al. [28]. In addition, when birth weight was lower than 1.000 g they were defined as extremely low birth weight (ELBW). Another way to classify newborns was based on gestational age: extremely preterm (EPT) when gestational age was lower than 28 weeks, very preterm (VPT) when gestational age was between 28–32 weeks, and late preterm (LPT) when gestational age was between 32–37 weeks.

A control group was established by recruiting children from a different observational epidemiological study made of an infant population (healthy full-term infants, Caucasian and children from Caucasian parents); the periodic evaluations followed the same patterns as those for VLBW infants (482 boys and 448 girls) [29].

An adequate catch-up growth in height or weight was defined when height or weight in VLBW infants, respectively, surpassed the value of 2 standard deviations below the mean in the growth charts of the control group [2–4].
