Catch up growth and its determinants in low birth weight babies: a study using Z scores.

Two hundred and forty seven low birthweight (LBW) survivors of our Neonatal Intensive Care Unit and 164 normal birthweight controls were followed up longitudinally from birth to 4 years and their growth trends (weight, height, head circumference) were expressed as mean Z scores in 500 g birthweight categories. Whereas LBW's demonstrated rapid growth in the first 6 months of life, followed by generally parallel trends with some tendency to rise, controls showed distinct growth faltering especially after one year. Only 30.8% of LBWs (and 49% of controls) were within the designated catch up levels for weight by age 4 years. The corresponding number for catch up of height and head circumference in LBW's was 22.8% and 26.5%, respectively. On multiple regression analysis, the most important determinants of catch up (at 4 years) in LBW's were weight at 1 year (beta = 0.51), height at 1 year (beta = 0.31) and mother's weight (beta = 0.04). Thus, Z scores enabled the demonstration of changing growth trends, simultaneous comparisons with local controls and international standards and comparison within indices. Growth charts incorporating Z score should be made available in a simplified manner for use in the community.