Purpose: During fetal development an increasing oxygen demandcan be supposed due to the growing cell mass. The regulatorymechanisms referring to this are still in discussion. Therefore theintrauterine time course of serum erythropoietin (sEPO) levelswas evaluated. Methods:Serum blood samples from 6 fetuseswith fetal alloimmune thrombocytopenia were obtained atdifferent gestational ages. Umbilical blood sampling wasnecessary to evaluate platelet count and was generally combinedwith intrauterine platelet transfusion. sEPO levels weredetermined by a commercially available ELISA-Kit (R&DSystems). Results: From the 22 to 24th, 25 to 28th, 30 to 34thand 37 to 40th week of gestation sEPO mean values were 8.7 ± 1.2 U/l, 9.5 ± 2.2 U/l, 16.7 ± 0.6 U/l and 24.6 ± 6.6 U/l, respectively. The corresponding mean values of the body weight determined by ultrasound parameters were 743 ± 130 g, 973 ±218 g, 2135 ± 516 g and 3550 ± 342 g. sEPO levels and body weight of the fetuses showed a linear regression (r = 0.7). Conclusions: In advanced gestational ages, elevated sEPO levels were observed, correlating with body weight of the fetuses. These changes of sEPO levels may be caused by the dynamic adjustment from paracrine to endocrine functions of the hormone.