Use of a body proportionality index for growth assessment of preterm infants

Abstract

Objective: To evaluate the utility of weight-for-length (defined as gm/cm(3), known as the "ponderal index") as a complementary measure of growth in infants in neonatal intensive care units (NICUs).

Study design: This was a secondary analysis of infants (n=1214) of gestational age 26 to 29 weeks at birth, included in a registry database (1991-2003), who had growth data at birth and discharge. Weight-for-age and weight-for-length were categorized as small (<10th percentile), appropriate, or large (>90th percentile).

Results: Statistical agreement between the weight-for-age and weight-for-length measures was poor (kappa=0.02 at birth, 0.10 at discharge; Bowker test for symmetry, P< .0001). From birth to discharge, the percentage of small-for-age infants increased from 12% to 21%, the percentage of small-for-length infants decreased from 10% to 4%, the percentage of large-for-age infants remained similar (<1%), and the percentage of large-for-length infants increased from 5% to 17%. At discharge, 92% of the small-for-age infants were appropriate or large-for-length, and 19% of the appropriate-for-age infants were large-for-length.

Conclusions: Weight-for-age and weight-for-length are complementary measures. Weight-for-length or other measures of body proportionality should be considered for inclusion in routine growth monitoring of infants in the NICU.

Figure 1. Discharge weight growth status of infants who were small weight-for-age at birth (n=149)

Small at Discharge (<10th percentile weight or length-for-age) Appropriate at Discharge (10-90th percentile or length-for-age) Large at Discharge (<10th percentile weight or length-for-age) Weight-for-gestational age (weight-for-age) and weight/length³ ratio-for-age (weight-for-length) based on Lubchenco fetal growth charts (Lubchenco Pediatrics 1966)

Figure 2. Discharge weight growth status of infants who were appropriate weight-for-age at birth (n=1062)

Small at Discharge (<10^th percentile weight or length-for-age) Appropriate at Discharge (10-90^th percentile or length-for-age) Large at Discharge (<10^th percentile weight or length-for-age) Weight-for-gestational age (weight-for-age) and weight/length³ ratio-for-age (weight-for-length) based on Lubchenco fetal growth charts (Lubchenco Pediatrics 1966)