It depends on what sort of information you are looking for, we recently experienced these problems in our Labour Ward, and it all came down to user issues, which were resolved by careful positioning of the transducers /switching to FSE, as checking the foetal and maternal heart rate independently.
The last MHRA guideline on this that I recall is
http://www.mhra.gov.uk/home/groups/dts-bs/documents/medicaldevicealert/con085077.pdf
