EN
To conduct the study two experiments were performed. In the first one the level of oestrone sulphate (EiS) and oestrone (Ei) was determined in the peripheral blood during the last two weeks before parturition. The mean Ei level was increasing progressively until the end of pregnancy. In contrast, the EiS values were at that time constant. However, just before parturition the concentration of EiS (15-18 ng per 1 ml) was 5-fold higher than that of Ei (3.0-3.5 ng per 1 ml). In the second experiment the level of the steroid hormones was measured in the uterine (umbilical vessels) circulation. The highest levels of EiS (approx. 35 ng per 1 ml) were found in the foetal blood (umbilical vein), while the highest concentration of Ei (approx. 8 ng per 1 ml) was in the uterine circulation (uterine vein). The data obtained suggest that placental oestrogens are primarily produced in the conjugated form (EiS) by the foetal part of the placenta; in the maternal placenta they are converted into unconjugated oestrogens form, in the perinatal period has been discussed.