Usually in the world of Omegaverse everyone can get pregnant depending on the pairings fertility rate. If it's to low possibility is zero, when high it's more likely to get pregnant.
Just think it is like this when reading Omegaverse:
Alpha x Alpha/Omega(heat)= 100%
Alpha x Beta = 75%
Beta x Beta = 50%
Beta x Omega = 25%
Omega x Omega = 1%
1 example difference: @Alpha bite. When Alpha bite omega nape they'll become a pair and the omega heat totally disappear. (Other Alphas can't smell pheromones anymore).
On another story it's so that the omega still can experience heat even after get bitten.
There are more variants depending on how to author wants it to be.. but that's to complicated to explain (@.@) So better not think much about it. You'll get crazy haha
Omegaverse just based from werewolf stories (That too got different types of settings). (IRL Wolves are also having Alpha, Beta, Omega hierarchy. Of course no such thing as everyone can give birth (≧∀≦))
I've read every Omegaverse available here. Here's my list if interested:
http://www.mangago.zone/home/mangalist/210068/
Does that mean that Miyauchi can be pregnant too? He's a beta, right? (⊙…⊙ )