In the Czech Republic since 1950 the birthweight of all children born and those who died within the first year is recorded. The modified definition of the term liveborn valid in 1951 till 1964 had a considerable influence on the structure of liveborn infants and the mortality rate in the first years of life recorded by birth weight. Therefore the changes were evaluated only since 1965. In the presented paper the authors draw attention to the possible effect of demographic, socioeconomic and genetic factors on birthweight in the population. Attention was also focused on the increasing incidence of deliveries from unmarried couples and the increasing number of deliveries of twins. The structure of liveborn infants by birthweight however did not change practically since 1965. Since the mid-seventies the infant and neonatal mortality of all children is declining, in recent years there is also a striking decline of the low birthweight infant mortality which is, no doubt, due to the significant advances achieved in perinatology. Throughout the period since 1965 the decline of the infant and neonatal mortality was greater in infants with a birthweight above 2499 g than in smaller infants.

        Key words: liveborn, birthweight, neonatal mortality, infant mortality, Czech Republic