The shortage of obstetricians and gynecologist is a serious problem in developed countries. To improve the workflow, we have been developing a robotic platform for supporting the fetal echo sonography, ``Tenang'', which allows for the ultrasound scan following the abdominal surface of pregnant women passively. Meanwhile, the acquired image quality was not constant due to the uncertain contact force between the probe and abdomen. In this paper, we aim to investigate the cause of the decreased image quality through a dynamic model analysis and experiment with a PVC phantom, and to propose the scan method which allows for the stable image acquisition. Thorough the investigation, the relative angle between the probe and body surface caused the contact force and acquired image quality. We hypothesized that the contact force and acquired image quality can be stable by maintaining the relative angle is constantly positive. Then, we proposed a spiral ascending scan that the probe constantly rises on the abdomen spirally. The proposed scan method is verified with an abdominal phantom in the second trimester. The results showed that the variation of the contact force under the spiral ascending scan path was reduced and the image quality was maintained.
