It has been established that small for gestational age (SGA) infants have a higher risk of developing chronic disease in adulthood. Our hypothesis is that SGA have a different early extrauterine growth rate than adequate for gestational age (AGA) infants. The main purpose of our study is to determine if there are differences in the early growth patterns of SGA vs AGA infants born of very low birth weight. The determination of the differences in the growth rate between SGA and AGA infants can lead to the development of nutritional guidelines directed to the needs of SGA infants and the reduction of health risks later in life.
Methods We performed a statistical analysis of a nested case control. Two hundred sixteen infants born of very low birth weight at the University Pediatric Hospital between 1999 and 2003 who survived to discharge were included. Patients with gastrointestinal conditions, birth defects, and those who were transferred to another institution during the first week of life were excluded from the study. The group of analysis was derived by pairing all the SGA infants with AGA infants by sex, year of birth, and birth weight (within 100 g). Data were obtained from the Vermont Oxford Network forms. Growth rate was defined as grams gained per kilogram of birth weight per day. We used two-sample t-test to determine the difference in growth rate between groups, and x2 for the evaluation of categorical variables. Odds ratio was determined to assess the impact of morbidity elements. Simple regression was used to establish the effect of morbidities on growth rate.
Results The mean birth weight for all the infants was 1,105 g (± 223 g), mean gestational age was 30 weeks (± 2.7 weeks), and mean growth rate was 13.4 g/kg/d (± 6.8 g/kg/d). The mean growth rate for AGA was lower (11.9 g/kg/d ± 7.6) than that of SGA infants (14.9 g/kg/d ± 5.5), resulting in a statistically significant difference in the extrauterine growth rate of AGA and SGA infants (p = .0000). Odds ratio analysis showed a higher risk of bronchopulmonary dysplasia (OR = 0.3), low APGAR (OR = 0.4), and sepsis (OR = 0.4) for the AGA group. When all variables were analyzed using the lineal regression model, only having a low APGAR score (p = .02) and being SGA (p = .0004) produced a difference in the growth rate of statistical significance. We conclude that the growth patterns of SGA and AGA infants are different and that the disparity in growth rate is not explained by the differences in the incidence of morbidities that affect extrauterine growth. Other elements, such as differences in the utilization of nutrients by SGA infants, should be evaluated in the future.