Pulse oximetry and oxygen saturation monitoring improve the medical care methods such as anesthesia and infants’ intensive care. Pulse oximetry method is basically more efficient than monitoring the fetal heartbeat since direct oxygen saturation measurement from fetal blood would be possible. The designing of device, including choice of an appropriate wavelength and space for monitoring, plays a significant role in improving its sensitivity to the depth and arterial pulse rate. This study simulates the interaction between light and tissue based on MONTE Carlo’s method and uses the results to construct the pulse oximeter.Method: For this purpose, a 10-layered tissue model, with different sizes of thickness, including artery pulses of systole and diastole phases, has been considered. Also, this study focuses on the separation of information related to mother and fetus.Result: According to the best location of detectors and trial and error method, the optimal wavelengths of 675 and 800 nanometers have been achieved. Studying only the single effect of each one of (shot and electronic noises) on the model shows the significant difference between the percent of input oxygen saturation and outcome results. However, considering these noises simultaneously causes less difference. The SIMULATION results are obtained in different wavelengths, using 106 photons and 49 detectors. The percentage of oxygen saturation, in comparison with the input database, has exhibited 2% error deviation from zero.