On the basis of retrospective analysis of 1913 deliveries (40-weeks, single, from healthy mothers) that took place in Bialystok hospitals from 1 January to 31 December 1997, the influence of sex and selected paragenetic and environmental factors on the somatic development of newborns was assessed. Mathematical description of the material was based on the following statistical methods: arithmetic mean (x), standard deviation (SD). In the assessment of significance of the obtained differences Student T test was used. The layout of empirical distribution curves of the examined parameters formed centile nets for the first-born newborns. The analyses revealed that the influence of the genetic factor manifested by sexual dimorphism is the highest in case of body mass at birth. The pathogenic factors of the strongest influence included pregnancy and delivery. Age of parents had smaller effect on the somatic development of the foetus. The influence of the mother's puberty time on the somatic development of the foetus is an interesting phenomenon. Women who mature earlier give birth to bigger newborns compared to those with late puberty time. As far as the environmental factors are concerned, no influence of education level or place of residence of the parents on somatic condition of the foetus was observed.