A leadless pacing device configured to deliver atrio-synchronous ventricular pacing, the leadless pacing device comprising: a plurality of electrodes (34, 36); a motion sensor configured to generate a motion signal as a function of movement of a heart of a patient; a stimulation module coupled to the plurality of electrodes, wherein the stimulation module is configured to generate pacing pulses and deliver the pacing pulses to a ventricle of the heart via the plurality of electrodes; an electrical sensing module coupled to the plurality of electrodes, wherein the electrical sensing module is configured to detect depolarizations of the ventricle within a cardiac electrogram sensed via the plurality of electrodes; a mechanical sensing module coupled to the motion sensor and configured to: receive the motion signal from the motion sensor; identify an activation of the ventricle; and characterised by the mechanical sensing module being further configured to: upon identification of the activation of the ventricle, initiate an atrial contraction detection delay period; analyze the motion signal within an atrial contraction detection window that begins upon completion of the atrial contraction detection delay period; and detect a contraction of an atrium of the heart based on the analysis of the motion signal within the atrial contraction detection window; a processing module configured to control the stimulation module to generate a pacing pulse and deliver the pacing pulse to the ventricle via the plurality of electrodes in response to the detection of the contraction of the atrium by the mechanical sensing module; and a housing configured to be implanted within the ventricle, wherein the housing encloses the motion sensor, the stimulation module, the electrical sensing module, the mechanical sensing module, and the processing module. a plurality of electrodes (34, 36); a motion sensor configured to generate a motion signal as a function of movement of a heart of a patient; a stimulation module coupled to the plurality of electrodes, wherein the stimulation module is configured to generate pacing pulses and deliver the pacing pulses to a ventricle of the heart via the plurality of electrodes; an electrical sensing module coupled to the plurality of electrodes, wherein the electrical sensing module is configured to detect depolarizations of the ventricle within a cardiac electrogram sensed via the plurality of electrodes; a mechanical sensing module coupled to the motion sensor and configured to: receive the motion signal from the motion sensor; identify an activation of the ventricle; and characterised by the mechanical sensing module being further configured to: upon identification of the activation of the ventricle, initiate an atrial contraction detection delay period; analyze the motion signal within an atrial contraction detection window that begins upon completion of the atrial contraction detection delay period; and detect a contraction of an atrium of the heart based on the analysis of the motion signal within the atrial contraction detection window; receive the motion signal from the motion sensor; identify an activation of the ventricle; and characterised by the mechanical sensing module being further configured to: upon identification of the activation of the ventricle, initiate an atrial contraction detection delay period; analyze the motion signal within an atrial contraction detection window that begins upon completion of the atrial contraction detection delay period; and detect a contraction of an atrium of the heart based on the analysis of the motion signal within the atrial contraction detection window; upon identification of the activation of the ventricle, initiate an atrial contraction detection delay period; analyze the motion signal within an atrial contraction detection window that begins upon completion of the atrial contraction detection delay period; and detect a contraction of an atrium of the heart based on the analysis of the motion signal within the atrial contraction detection window; a processing module configured to control the stimulation module to generate a pacing pulse and deliver the pacing pulse to the ventricle via the plurality of electrodes in response to the detection of the contraction of the atrium by the mechanical sensing module; and a housing configured to be implanted within the ventricle, wherein the housing encloses the motion sensor, the stimulation module, the electrical sensing module, the mechanical sensing module, and the processing module.