A retrospective study on the growth trajectories and nutritional support of preterm infants admitted to the Neonatal Intensive Care Unit at Dunedin Hospital

Abstract

Introduction Premature infants are at high risk of undernutrition and extrauterine growth restriction (EUGR) and thus, monitoring of growth is crucial for optimizing the health and well-being of this vulnerable population group. The newly published INTERGROWTH-21st Preterm Growth Standard released in late 2015 provides the first prescriptive growth trajectory of how an infant should grow however, it has not yet been implemented clinically. Objectives Thus, the aim of the present study is i) to determine if differences in growth z-scores and risk of undernutrition among preterm infants born < 33 weeks gestation exist between the INTERGROWTH standard and commonly used NZ growth reference charts; and ii) to assess the growth trajectory of a sample population of preterm infants < 33 weeks gestational age admitted to a New Zealand tertiary care facility using the INTERGROWTH standard, and evaluate the relation between feeding practices and these growth outcomes from birth to approximately 48 weeks Post Menstrual Age (PMA). Results and Discussion Longitudinal data were retrospectively collected on infants born < 33 weeks gestation admitted to Dunedin NICU between January 2013 and June 2015. Weight, length and head circumference at birth, 28 days of life (28-DOL), 36 weeks and 48 weeks PMA were collected, z-scores were calculated using the INTERGROWTH Preterm Growth Standard, and commonly used UK-90 and Fenton Preterm Growth References. A number of different classifications were used to define insufficient growth including the proportion of infants classified with a z-score of < -1.28 SD (less than the 10th chart centile), z-score ≥ -2 & <-1 SD) often used to assess ‘at risk’ and a z-score SD <-2 used to define stunting and underweight. McNemar chi-squared test and paired t-tests were employed to assess whether significant differences in classification existed between the INTERGROWTH standard and the growth reference charts. Growth velocity (g/kg/day) was determined via an exponential model from birth to 36 weeks PMA and the post-discharge period from 36 weeks PMA to 48 weeks PMA. Nutritional practices including enteral nutrition, withholding of feeds, nutrient intake at 36 weeks PMA and feeding at discharge were described. Multiple regression was used to explore associations between growth outcomes and nutritional variables. In total, data were collected on 103 preterm infants. Mean (SD) gestational age of infants was 29.1 (2.5) weeks and birth weight was 1290 (403) g. A comparison of the growth using the different charts revealed significant z-score differences, specifically for weight z-score ≥ -2 & <-1 at birth, head circumference at birth across all categories, and length z-score < -1.28 SD at 36 weeks PMA. Overall, mean z-scores determined using the UK-90 were significantly lower for all growth measures at birth and 36 weeks PMA compared with the INTERGROWTH (all, P<0.001) with the exception of weight-for-age at 36 weeks PMA An overall evaluation of preterm growth using the INTERGROWTH standard revealed that the prevalence of growth faltering (weight-for-age z-score <-1.28) increased from 9% at birth to 13% at 28-DOL, 19% at 36 weeks and 30% at 48 weeks PMA. Mean (SD) GV in-hospital [14.2 (3.3) g/kg/d] declined substantially post-discharge [8.2 (1.7) g/kg/d], falling well below the desirable rate of 18g/kg/d. Weight at 28-DOL was a significant predictor of improved growth through to 48 weeks PMA. Fortified human milk in hospital was positively associated with length at 36 weeks PMA while reported feeding of infant formula at discharge was associated with improved head circumference at 48 weeks PMA. Conclusion Differences were evident between the INTERGROWTH Preterm Growth Standard, with the UK-90 exhibiting the most pronounced differences and misidentification of infants at risk of poor growth. Secondly, EUGR was highly prevalent in this sample population of preterm infants. Optimal growth in the first month of life appears critical for the support of desirable postnatal growth outcomes and reinforces the need for early nutritional support. In addition, a greater emphasis on post-discharge nutrition and growth monitoring is warranted in this sample population.