An ultrasound processing unit, for use in fetal monitoring for distinguishing different heart rate sources within received Doppler ultrasound data, the ultrasound processing unit being communicatively coupleable in use with at least two ultrasound transducer sources; and the ultrasound processing unit configured to: receive first input Doppler ultrasound data from a first ultrasound transducer source, and receive second input Doppler ultrasound data from a second ultrasound transducer source; compile from the first and second input Doppler ultrasound data a single set of input ultrasound signal channels, each corresponding to a different particular tissue region within the subject; perform a principal component analysis, PCA, procedure, configured to identify one or more linear combinations of the input signal channels which are statistically uncorrelated, the linear combinations defining, when composed, a set of first output signals, and perform an independent component analysis, ICA, procedure, configured to identify one or more linear combinations of said first output signals which are statistically independent from one another, said one or more linear combinations defining a set of one or more second output signals, the one or more second output signals thereby providing signals corresponding to distinct heart rate sources. the ultrasound processing unit being communicatively coupleable in use with at least two ultrasound transducer sources; and the ultrasound processing unit configured to: receive first input Doppler ultrasound data from a first ultrasound transducer source, and receive second input Doppler ultrasound data from a second ultrasound transducer source; compile from the first and second input Doppler ultrasound data a single set of input ultrasound signal channels, each corresponding to a different particular tissue region within the subject; perform a principal component analysis, PCA, procedure, configured to identify one or more linear combinations of the input signal channels which are statistically uncorrelated, the linear combinations defining, when composed, a set of first output signals, and perform an independent component analysis, ICA, procedure, configured to identify one or more linear combinations of said first output signals which are statistically independent from one another, said one or more linear combinations defining a set of one or more second output signals, the one or more second output signals thereby providing signals corresponding to distinct heart rate sources. receive first input Doppler ultrasound data from a first ultrasound transducer source, and receive second input Doppler ultrasound data from a second ultrasound transducer source; compile from the first and second input Doppler ultrasound data a single set of input ultrasound signal channels, each corresponding to a different particular tissue region within the subject; perform a principal component analysis, PCA, procedure, configured to identify one or more linear combinations of the input signal channels which are statistically uncorrelated, the linear combinations defining, when composed, a set of first output signals, and perform an independent component analysis, ICA, procedure, configured to identify one or more linear combinations of said first output signals which are statistically independent from one another, said one or more linear combinations defining a set of one or more second output signals, the one or more second output signals thereby providing signals corresponding to distinct heart rate sources.