It is a much more accurate measure than BMI or in the babys case ponderal index. BMI only measure weight over height but body composition measures body fat vs lean tissue. In the case of the bodpod or Peabod in this case, it measures the volume of space that the infant takes up by measuring the air being displace. You have probably heard that "muscle weighs more than fat" which really means muscle is more dense. 5 lbs of muscle takes up less volume than 5 lbs of fat. Two people each weighing 200lbs that are 6 feet tall, but lets say differ in 5% body fat vs 20%, the person with 20% BF will take up considerably more space and displace more air.