Antigorite is the high-temperature member of the serpentine group minerals and is broadly considered a primary carrier of water in subducting oceanic lithosphere at the fore-arc. It has a wavy crystal structure along the a-axis and several polysomes with different $m$-values (m=13-24) have been identified in nature. The number (m) is defined as the number of tetrahedra in one wavelength and is controlled by the misfit between the octahedral and tetrahedral layers. The degree of misfit mainly depends on the volumes of the MgO6 octehedra and SiO4 tetrahedra within the layers, which vary as a function of temperature and pressure. However, it is not well understood which m-values of antigorite are stable at different temperature and pressure conditions. To investigate the pressure dependence of the stability of different m-values in antigorite, we performed first-principles calculations for several polysomes (m=14-19) at high pressure (0-14 GPa) and compared their enthalpies (T: static 0 K). We found that although the energy differences between polysomes are small, polysomes with larger m-values are more stable at ambient pressure, while polysomes with smaller m-values are more stable at elevated pressures. This suggests that the structure of antigorite in oceanic lithosphere that has subducted into the deep Earth may gradually evolve into a different polysome structure than antigorite samples observed at ambient or near-pressure conditions. This structural change may be related to the formation of the lower plane of the double seismic zone, as changes in polysome m-values are accompanied by a minor dehydration reaction.