Purpose The shortage of obstetricians and gynecologists has intensified in developed countries. Our long-term goal is to develop a robotic prenatal care platform for automatic ultrasound (US) scanning to improve the workflow efficiency of obstetricians and gynecologists. This paper develops a hardware platform for the positioning of the US probe to obtain diagnostic US images while satisfying safety requirements of the fetus and pregnant woman. Method The proposed system includes a mechanism that maintains the contact force in a certain range and passively adjusts the US probe posture relative to the body surface. The system is designed according to clinical survey data. For proof of concept, we conducted a robotic US scan with an agar phantom and three pregnant women under the operation of a physician. Results Experimental results show the passive US scan motion followed the phantom surface with an acceptable contact force (< 15 N). Clinical trials were safely carried out with observations of fetal body parts. Conclusion Our proposed platform acquired US images with satisfactory contact forces in the phantom study. The feasibility of the platform was demonstrated in a clinical study.