Weakly first order transitions from a disordered state to periodic ordered phases, are observed in many physical systems, such as diblock copolymer melts, weak crystallization in liquids and micellar systems. In the weak segregation regime, successful theories have used a Landau-Ginzburg form of the free energy. It is well established that a complete description of the system necessitates the inclusion of thermally induced concentration fluctuations in the theory. In some cases it may also be important to construct the density in the periodic structure from higher harmonic vectors other than simply the first ones. Recently, exotic structures have been observed experimentally which cannot be distinguished from simple morphologies unless higher harmonics and their phases are included. In this paper we present a general theory where the order parameter is expanded up to the second harmonic vectors with phase factors, and where the concentration fluctuations are included self-consistently in the analysis.