During its propagation in a rock a fracture may cross mechanical heterogeneities, which modify the stress field near the crack tip and therefore may affect the direction of propagation. Pre-existing strong (grains) and weak (pores, microcracks) defects control the final path of the fracture and the amplitude of its out-of-plane fluctuations; they may also control rupture arrest. In situ quantification of the role of heterogeneities on fracture propagation is challenging because of the technical difficulty to image the interior of a 3D medium at high spatial resolution. Here, hydraulic tension fractures were produced in 5% porosity limestone core samples, using a specially designed hydraulic cell. The 3D geometry of the centimeter-scale samples was imaged before and after fracturing, using X-ray computed synchrotron microtomography at a voxel resolution of 4.91×4.91×4.91 μm. The data show that hydraulic fractures propagated by linkage of pores, leading to a macroscopic fracture with well-developed roughness. Moreover, it was possible to estimate that the hydraulic fractures crossed up to 40% more heterogeneities (pores) than if they had propagated into the porous medium by randomly connecting these pores. This demonstrates and quantifies the strong control of local mechanical variations on rupture propagation. A statistical model of fracture propagation is proposed, involving linkage of nearest pores; this model quantitatively reproduces our experimental observation.< Réduire