The relation between weight and age in the two years following conception describes a gentle S-shaped curve. Between 28 weeks' gestation and 18 weeks' postnatal age the relationship appears linear, indicating a uniform incremental weight gain. The linear part of this curve has been used to help select an arbitrary reference line to reflect the normal average growth of a healthy population during this period. The flattening in the fetal growth curve usually observed after 40 weeks' gestation is considered to be due to the constraint of fetal growth and is therefore ignored. Arguments are advanced for believing that there is a normal distribution of weight around the central tendency at this time, and that, for practical purposes, normally growing fetuses and infants have an almost identical potential growth velocity during this period and differ for the most part only in the weight they have achieved at 28 weeks' gestation. Using these assumptions a versatile reference chart for international use has been created that is easy to remember and to construct. Allowance may be made for biologic variables such as sex and maternal height. It may be used to categorize groups of infants, to study longitudinal growth following delivery, as well as to compare different populations.