Non invasive foetal electrocardiogram (ECG) extraction involve measurements from electrodes located on different points of the mother's skin. As a result, foetal ECG (fECG) and maternal ECG (mECG) components appear mixed together in the recordings, in addition with some random disturbances, such as mother electromyograms, thermal noise from the electrodes,etc. Fetal contribution however is minor by comparison with these electrical sources and classical signal processing, like filtering, does not allow us to recover the fECG. The task of obtaining the fECG from the rest of non-desired signals, that disturb the cutaneous recordings, can be coped by using blind source separation (BSS) tools. In the following, we propose some conditions on the number and location of the sensors so that the extraction is optimal.