Omegaverse (or ABO) is an AU (alternate universe) in which males can get pregnant. In this system, there are three main roles: Alpha, Beta, and Omega.
Alpha/Beta/Omega or Alpha/Omega (occasionally Alpha/Beta) is a kink trope wherein some, or all people, have defined biological roles based on a hierarchical system, with the terms originating from animal behavior research. There may be a werewolf, knotting, or other animalistic elements involved, or the characters may be otherwise purely human.
M-Preg (or Male pregnancy) is different from Omegaverse. Omegaverse can contain m-preg but not all m-preg is omegaverse.
Within the M-preg world is the ‘ger’ trope. The gers are males who can get pregnant and consider themselves men. They are in a body that is pretty much (aside from the womb, which is internal) pure male. Most gers have a red "cinnabar" dot somewhere on their body to indicate that they are indeed a ger instead of a regular male. They are almost like an omega without estrus/heats and pheromones or marking. These mostly contains novels in ancient times and gers may be referred as 'bio', 'moye', or other names.