1. A method of determining the oxygen saturation of fetal blood, the method comprising emitting light of a first wavelength onto the abdominal surface of a pregnant mammal over time; emitting light of a second wavelength onto the abdominal surface of the preganant mammal over time, wherein the second wavelength is different from the first wavelength; detecting the intensity of light scattered by fetal and maternal arterial blood at the first wavelength and light scattered by fetal and maternal arterial blood at the second wavelength over time, wherein the time of detection is sufficient to record at least one fetal heart beat and at least one maternal heart beat; converting the detected light into electronic signals; processing the electronic signals to separate maternal and fetal contributions to the electronic signals; removing the contributions to the electronic signals generated by maternal arterial blood; measuring the absorption of light at the first and second wavelengths by fetal arterial blood; and calculating the oxygen saturation of the fetal arterial blood from the measured absorption of light over time.