INTRAUTERINE GROWTH IN ACTIVE TISSUE MASS OF THE HUMAN FETUS, WITH PARTICULAR REFERENCE TO THE UNDERGROWN BABY

Abstract

Oxygen consumption of babies born after various lengths of gestation, measured between 2 to 10 days of post-natal life under resting, thermoneutral conditions, has been used as an index of intrauterine growth in "active tissue mass." A curve was constructed describing the median increase in oxygen consumption with increasing gestational age in babies who had experienced usual rates of growth in utero. Babies who have grown normally in utero exhibit, with increasing birth weight and with increasing gestational age, an increase in oxygen consumption per kilogram birth weight. This phenomenon is proposed to result from changes in fetal body composition with increasing maturity whereby the cell mass constitutes an increasing percentage of total body weight as length of gestation increases. Babies who have been undergrown in utero consume, as a group, more oxygen per kilogram body weight than do normally grown babies of similar birth weight; the degree of hypermetabolism is correlated with the degree of undergrowth. Undergrown babies, as a group, do not have a higher oxygen consumption per kilogram than normally grown babies of similar gestational age, although there is a tendency for those most undergrown to be hypermetabolic even for duration of gestation. The relative hypermetabolism of undergrown neonates is proposed to depend on a dual etiology: a cell mass/body weight ratio characteristic of maturity rather than size, and, in the most undergrown subjects, an increased cell number/cell mass ratio as would occur with reduction in cell size. A model is developed which relates observations by others of changes in organ size and cell size, in malnutrition, to the present metabolic measurements. Additional observations include external dimension—oxygen consumption relationship, colon—skin temperature gradients, and acid-base parameters, in babies either normally grown or undergrown in utero. Further study is needed of increments in oxygen consumption during the neonatal period in these two kinds of babies.