We study transitions to periodic structures in diblock copolymer melts including self-consistently the concentration Ructuations, and expanding the density in multiple harmonics. The fluctuations induce a shift in the maximum scattering wave vector in the isotropic state which suggests that the chains stretch near the transition. Transitions to 3D hexagonal lattices, 2D hexagonally packed cylinders, and bcc structures are predicted. We find the periodicity near the transition: The chains contract in the bcc, while they stretch in the other structures