Oxygen is carried in red blood cells as hemoglobin (Hb) and is one of the key vital signs that needs detection. Pulse oximetry takes advantage of the fact that blood absorbs certain wavelengths of light differently when oxygenated than when oxygen deprived. Transmission and reflectance are the two main modes of pulse oximetry. In transmission mode, a probe is typically placed on the finger to measure light absorption in the light frequencies of interest. In obstetrics, due to the absence of a transilluminable fetal parts for transmission oximetry, the only feasible option is the reflectance mode, in which the sensor and detector are located on the same surface of the body part.