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Steriodogenesis plays a key role throughout the course of pregnancy.The function of steroidogenesis in the placenta has not yet been studied in detail, however, especially the interactions between the placenta, embryo, and mother.New information has recently appeared about the precursors of some active steroids coming from the embryo, as compared to generally acknowledged opinion that they originated from the mother.Investigated material from newborn umbilical cord blood is a source of important information on the function of the placental, fetal and maternal compartments.A precise view of the activity of these compartments can be obtained from selective analyses of arterial and venous cord blood.Selective sampling of arterial and venous cord blood is technically difficult, however, and during premature deliveries the umbilical vessels quickly collapse and separate blood collections are often extremely technically demanding.The aim of this work was to develop a model from which the concentrations of individual steroids can be extrapolated from mixed cord blood.Umbilical cord blood from 50 newborns was separately collected from arteries and veins.Twelve of these were physiological births (the 40th week ± 2 weeks) and 38 were premature deliveries due to maternal indications.The steroid spectrum reflected the week of pregnancy.In all samples, 69 steroids were measured along with their polar conjugates using gas chromatography-mass spectrometry, and the arterial-venous differentiation of steroids from cord blood was analyzed.The main steroid levels from venous and arterial cord blood were described for individual weeks of pregnancy from the 24th week.Then, a model was developed of the different steroid spectra between venous and arterial cord blood.This model allows the extrapolation of steroid levels from mixed blood to respective venous and arterial levels, which in the future could simplify the technical aspects of cord blood sampling during studies of steroidogenesis, especially in premature births where separate sampling is technically very difficult.Following the changes in steroidogenesis during premature births arising from the side of the embryo may give us insight into their causes.Our model should simplify such investigations.