Oxycardiotocography is a combination of cardiotocography and continuous registration of the foetal arterial oxygen saturation (SaO2). In a few cases, the value of this additional information of foetal SaO2 is demonstrated. During uncomplicated deliveries with normal foetal heart rate patterns, the foetal SaO2 usually ranges between 50% and 70%. Uterine hyperactivity with impaired perfusion of the placenta shows besides a changed heart rate patterns, a significant decrease of the foetal oxygen saturation. Conversely, by supplying oxygen to the mother with a mask, the foetal oxygen saturation can be increased by approx. 10%. In case of a breech presentation and application of the sensor to the buttocks, the measured O2-saturation is approx. 10% to 15% below the values usually obtained from the scalp because the tissue there is supplied with mixed blood after the ductus.