A method for processing cardiac signals includes accepting, from a sensor system, a set of one or more signals, the signals including components of a desired cardiac signal and components of a substantially periodic interfering signal. The method is applicable for extraction of desired fetal cardiac signals from signals with interference from the maternal cardiac signal. A periodicity of the interfering signal is determined, and one or more iterations of mitigating an effect of a component of the signals that exhibit periodicity at the determined periodicity of the interfering signal are performed. In some examples, the method further includes determining a periodicity of the desired cardiac signal, and performing one or more iterations of enhancing an effect of a component of the signals that exhibit periodicity at the determined periodicity of the desired cardiac signal.