Fetal well-being depends on the level of oxygenation in vital organs such as the heart and brain. In this review, we discuss the physiological parameters which underlie the use of pulse oximetry to evaluate fetal conditions intrapartum. Whilst the measurement of haemoglobin oxygen saturation (SaO2) depends on partial pressure of oxygen (PO2), the relation is alinear, is relatively insensitive to changes in PO2 at the upper physiological range, and it is affected by the Bohr shift. Oxygen content of the blood cannot be determined without measurement of haemoglobin (Hb) content and this can change quite quickly. In hypoxia for example, oxygen delivery to an organ, e.g. the brain, cannot be assessed without simultaneous measurement of blood flow, which again changes with fetal condition. Lastly, it is not possible to gauge fetal tissue unless some measure of, for example, cytochrome aa3 oxidation is used: tissue oxidation in relation to oxygen delivery can change due to local vascular readjustments and changes in metabolism. We conclude that use of SaO2 to assess fetal well-being is fraught with difficulties, and that much more research is needed before its routine clinical use can be considered.