Apparatus for monitoring respiratory and/or convulsive movements includes a closed loop phase lock system which measures changes in the air path length of continuous wave ultrasound transmitted to and reflected from (the chest/abdomen area of) a subject, and provides an output change signal proportional to changes in air path length caused by respiratory effort of the subject. A window/alarm circuit receives the change signal and triggers an alarm if the change signal remains below a preset lower threshold for a selected time interval, as occurs in apnea. An upper threshold may be set and an alarm triggered if the change signal exceeds the upper threshold for a selected time interval, as occurs in the case of convulsions. The apparatus includes means to provide a signal which follows movement of the subject in an unambiguous manner and which will not trigger an alarm due to extraneous (non-subject) movement. A method of monitoring movement of a subject is provided which includes transmitting continuous wave ultrasound from a source to a subject and receiving the reflected ultrasound. The method includes measuring changes in the air path length of the standing wave and providing an output signal which is proportional to the magnitude of the changes and sounding an alarm when the output signal falls outside a preselected range for a predetermined period of time.