Abstract: In order to replace the conventional method of vagina palpation which has been widely used in obstetrics,a new method for detecting fetal head position and cervical dilation automatically is proposed based on the analytic geometry of space and ultrasonic triangulation.A tracking (position) model is set up and kinds of factors which may cause detecting errors are discussed.A ultrasonic measurement system for body tracking (position) is developed.The error of average result from 5 times detection by this system is less than 0.25 cm,much less than the error of a conventional vagina palpation,that is average around 2.0 cm.