The change in backbone conformation from prolate to oblate upon going from the re-entrant nematic to the smectic Adphase is studied in a comb-shaped polyacrylate by measuring the mosaicity of the sample in the smectic phase with X-ray scattering. In combination with the possibility of aligning the director in both nematic phases, the results indicate that the conformational change takes place in the re-entrant nematic phase upon approaching the smectic Adphase.