There is no mention of menustration in the omega verse, but in reality, canines (of which the omega verse is loosely based) do not menustrate because they are only sexually/reproductively responsive during their heat cycles as opposed to humans who go through a cycle (1 month) of creating an egg and a hospitable environment and destroying it (menustration) if it doesn't get fertilised. By this logic, an omega would create an egg only during its heat cycle and only if they are accompanied by an alpha. That's probably why most stories aren't concerned with pregnancy outside the heat cycle.