A pneumatic tocodynamomeer ("pTOCO") (10) having a guard-ring (13) with a thin elastic membrane (18) stretched across a shallow spherical depression (12) trapping a small volume of air in the centre of the guard-ring (13). A pressure transducer (17) may be moulded into the body (11) of the pTOCO (10). In an alternative embodiment, the air volume beneath the membrane (18) of the pTOCO (10) may be connected via a low volume air conduit (14, 15) to a separate pressure transducer (16).