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The results of the study of the length of the prenatal period of ontogenesis of 51 Ukrainian Red Dairy breed tribal cows are presented, and the relationship of this trait with the rate of growth of the organism in the postnatal period (at birth, at 6, 12 and 18 months) and dairy productivity for the first lactation are investigated. Also, at different stages of the prenatal period of ontogeny the dynamics of the mass of 66 fetus and some parenchymatous organs (liver, spleen, thymus) selected by anatomical preparation are determined. The aim of the research was to establish the possibility of predicting dairy productivity of Ukrainian Red Dairy breed cows over the length of the prenatal period and the effect of this period on the postnatal growth rate of the organism, and also to ascertain the existence of a regularity between the increase in fetus mass and the development of certain hemopoiesis organs at different stages of prenatal development. Laboratory animals were divided into three groups, depending on length of their prenatal period: less than 277 days-reduced (group I), 277-283 - medium (group II) and more than 283 days - extended period (group III). For 305 days of the first lactation, the advantage over milk yield, milk fat yield and milk protein belongs to animals with shortened duration of the prenatal period of ontogenesis in comparison with contemporaries who had an extended period with a statistically significant difference (P < 0.05). A further study in the short term may be a study of the relationship between the duration of the prenatal period and the rate of fetal growth with the subsequent forming of the physique, and in the long term turning into the study of the relationship between the duration of the prenatal period and the rate of fetal growth with a duration of productive use and lifelong productivity. [ABSTRACT FROM AUTHOR]