Objective US and MRI are two validated techniques for fetal brain expertise, each presenting their own advantages. Fusion imaging (FI) might result in one final merged image with the exactness of both modalities. The objective was to evaluate the clinical use of the off-line US-MRI FI in the assessment of the fetal brain development. Material and Methods This was a prospective study in Necker Hospital (NCT04142606) conducted from January to May 2021. We included normal singleton pregnancies between 20 and 36 weeks. Each woman has had a volumic 3D US and a T2w MRI brain exam (1.5 T) on the same day. Then, we merged US and MRI images with the dedicated 4D View Fusion software (GE Healthcare) by using precise landmarks to optimize the registration process. We developed a standardized method to identify 29 brain structures previously defined to provide an acute expertise the fetal brain development in FI. Results More than 100 patients were included and we obtained workable US and MRI acquisitions in the orthogonal planes allowing for fusion imaging in the vast majority of cases. Overall, FI gave access to a larger amount of information to establish a complete fetal neurosonogram than each modality by itself. At late gestational age, the examination of the parenchyma became very difficult due to fetal position, shadow cones and skull ossification. US resolution highlighted some median brain structures such as basal ganglia, trigone, Sylvius aqueduct or cerebellar foliation; On the opposite on the same plane, MRI gave ac- cess to the full brain gyration, brain stem or peri-cerebellar spaces. Moreover, a 3D Doppler acquisition merged to an MRI could provide some vascular information, so far unattainable with fetal MRI. Conclusion We present a promising off-line US-MRI FI tool allowing to navigate within two merged volumes acquired during the same day. This tool could be of major interest for educational purpose, brain development assessment and in case of fetuses at high risk for brain damages such as CMV infection or TTTS.