A detailed analysis of the impact of pitch variations in block copolymer self-assembly patterns on defect density and placement errors inside lithographically defined trenches is presented. The presence of random variations in the pitch of nanopatterns resulting from the self-assembly of cylindrical phase block copolymer in open trenches (one-dimensional confinement) is experimentally demonstrated and confirmed by theoretical modeling. The simulations show that the intrinsic pitch spread is not affected when the same block copolymer is studied inside closed trenches (two-dimensional confinement) where the observed spread has a direct influence on placement and defectivity. The magnitude of the detrimental impact of the pitch variations on both defect density and placement accuracy in closed trenches is strongly dependent on trench length. These findings pose new design rules for future applications of directed self-assembly in semiconductor industry.