Silicon-based photonic crystals (PCs) are a subject of increasing interest. The porous structures are electro-chemically etched into Si wafers, providing 2D triangular PCs of high dielectric contrast. These structures were inspected by scanning electron microscopy (SEM). The analysis of the SEM images proved that the pores, being originally considered as circular, reveal oval-shaped deformations. Main topics of the paper are band gap modifications (TM- and TE- polarization) for pores of deteriorating roundness, approximated by ellipses of varying eccentricity. In particular, the contribution presents theoretical results (plane wave approach), analysing the influence of the pore roundness on the maximum complete gap size. Two modes of pore growth are compared theoretically: (i) the major axis of all pores being etched is constant (eccentricity varied by area), (ii) the area of the pores is the process constant (eccentricity varied by elliptic axes). The eccentricity and the angular orientation of the pores in the lattice were systematically varied. We paid special attention to pore sizes quite near to that circular pores forming the maximum complete gap. It turned out that 2D triangular PCs having elliptical pores can not surpass the maximum gap size attainable by perfectly circular pores.