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To calculate regional fetal brain oxygen saturation (sO2) during hypoxia in sheep.Eight pregnant ewes were examined at a 3T MR-scanner using blood oxygenation level-dependent magnetic resonance imaging (BOLD-MRI) to measure signal intensity changes of the fetal brain during a control period and a period of induced hypoxia. Regions of interest were placed in the fetal cerebrum to assess ΔR2* from GRE signal intensity plateaus (Scontrol, Shypoxia) and the relation between ΔR2* and ΔpO2 was analyzed. A probe was placed surgically in the fetal brain to directly measure local pO2 as a reference standard. Baseline and hypoxic pO2 values were recorded and compared (ΔpO2).Mean fetal brain pO2 decreased from 14.3 mmHg (95% confidence interval [CI]: 10–19) to 3.4 mmHg (95% CI: 2–5) during hypoxia (mean ΔpO2 = 10.9 mmHg and ΔR2* = −5s−1). A significant correlation between ΔR2* and ΔpO2 was noted (r = 0.93, P < 0.001), and conversion of pO2 into sO2 resulted in a linear regression coefficient of (−0.14 ± 0.01)s−1/% (r2 = 0.91).Measured fetal brain BOLD-MRI was compared and converted to pO2, followed by calculation of cerebral sO2. J. Magn. Reson. Imaging 2015;41:110–116. © 2013 Wiley Periodicals, Inc.