Mendelian genetics showed that a few mutated genes, or errors in parental imprinting, can lead to major phenotypic changes (diseases) in pre-natal growth. Mendelian genetics, however, do not explain the individual subtle variability of size at birth within the normal range. Fetal growth is a complex multifactorial, multigenic trait made of various sub-traits, such as body mass, fat and muscle, brain mass, head circumference, skeletal growth of the spine and limbs. It is likely that multiple genetic factors and genomic variants are responsible for the variations of these sub-traits. A study has been launched to investigate the genetics of the variation of human birth weight, with the ultimate aim of identifying genomic variations that are within or near certain genes and are associated with variations of human height and weight at birth.