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OBJECTIVE: We evaluated fetal Erythropoietin (EPO) in a clinical obstetrical environment, aiming to collect more information upon its role in diagnosis of fetal stress. As it is known that EPO cannot pass the placental barrier, we restricted to measure the concentration in the umbilical artery (UA) immediately after delivery. PATIENTS AND METHODS: EPO was correlated to relevant obstetrical parameters. We looked for each parameter that was tested for differences in fetal sex. For this purpose, the statistics were performed by means of the Behrens-Fisher-Test (meaning a special application of the Student-T-Test). This device is an algorithm developed to compare regressions. For significance, a confidence level of less 5% was accepted. For the rest, we only used descriptive devices. RESULTS: We found weak but positive correlation to gestational age for both sexes. No correlation was calculated for the duration of parturition or the birth weight. Highly significant differences were found for respiratory values of blood gases, taken from the UA when differentiated for fetal sex: males showed a strong and negative correlation for pO2 (r(m)= -0.45); the female EPO-level was not influenced by decreasing O2-level (r(w)= +0.06, p > 0.01). Similar results could be found for pCO2. (r(m)= +0.36; r(w)= -0.07; p < 0.01. Not significant, but comparable differences for the EPO-level in fetal blood were found for pH and base deficit. CONCLUSIONS: If we accept that the EPO-level in fetal blood rises with relevant oxygen deficiency, then females seems to be better protected against damage from distress. This observation is congruent with the clinical experience from the authors who have the subjective impression that female sex is advantageous in harmful deliveries.