Recently, in the United States as well as other countries, a shortage of obstetrician and gynecologist (ob-gyns) has grown seriously. The obstetrics and gynecology have a high burnout rate compared to other medical specialties because of increased workloads and competing for administrative demands. Then, there is a demand for assisting the procedure of prenatal care, especially ultrasonography. Although several robotic-assisted ultrasound imaging platforms have been developed, there were few platforms focusing on prenatal care. In this paper, we proposed an ultrasonography assistance robot for prenatal care to improve the workload of obstetricians and gynecologists. In prenatal care, it is crucially important to satisfy the safety for the pregnant women and fetus compared to other regions of ultrasonography. This paper serves as the proof of concept of the ultrasonography assistance robot for prenatal care by demonstrating the scan of uterus and estimating amniotic fluid volume for assessing fetus health with the fetal US imaging phantom, and clinical feasibility to one pregnant woman. As the key technology to satisfy the safety and acquired image quality, the mechanism with constant springs that the US probe can be shifted flexibly depending on the abdominal height was proposed. The proposed robot system enabled to scan the entire uterus area keeping the contact force under the force applied in clinical procedures (about 15 N) to the fetus phantom. Additionally, as the first application for evaluating fetus health automatically, the system to estimate the amniotic fluid volume (AFV) based on the acquired US images with the robot system was developed and evaluated with the fetus phantom. The result shows estimation errors within 10%. Finally, we demonstrated the robotic US scan to one pregnant woman and successfully observed the body parts of fetus.