In forward osmosis, defects on the selective active layer and changes in the porous structure of the support layer can be detrimental factors affecting the membrane performance. This study focuses on the impacts of (i) the possible presence of defects and (ii) the changes in pore structures on the water flux and membrane selectivity via computational fluid dynamics analyses. Results suggest that diffusion of the draw solute through the support layer, i.e. internal concentration polarization, can be strongly enhanced or reduced by widening or narrowing the shape of the pore, respectively, while no significant effect on water permeation can be associated with the change in draw solution cross-flow velocity or in the loss of draw solute during filtration. Interestingly, defects within the active layer may affect the water flux exponentially as function of the defect size, suggesting the presence of a threshold below which the convective passage of contaminated water flux through the defect is not affecting the membrane productivity. Moreover, the presence of defects may not be a detrimental factor for membrane operating with high nominal rejection (>90%) and low percentage of defected area (<1%).