In this work, we use nonequilibrium molecular dynamics to simulate a contraction-expansion flow of various systems, namely melts with molecules of various conformations (linear, branched, and star), linear molecules in solution, and a reference Lennard-Jones fluid. The equations for Poiseuille flow are solved using a multiple time scale algorithm extended to nonequilibrium situations. Simulations are performed at constant temperature using the Nose-Hoover dynamics. The main objective of this analysis is to investigate the molecular origin of pressure drop along planar contraction-expansion geometry, varying the length of the contraction, and the effect that different molecular conformations have on the resulting pressure drop along the geometry. Pressure drop is closely related to mass distribution (in neutral and gradient directions) and branching index of molecules. Also, it is shown that remarkable increases of pressure drops are also possible in planar geometries, provided large extensional viscosities combined with moderate values of the first normal stress difference in shear are considered, in addition to considerable reductions of the flow area at the contraction region.