
It is difficult. Most of the stories I read feat this pairing, the pregnancies are considered to be some sort of miracle. Esp with Kashiko since apart from the uke being a beta, be has pregnancy issues because of the abuse he went through. It depends on the author prolly. Maybe the author of love is an illusion is hust teasing, in the future there will be a baby for your alpha beta pairing.

Omegas are the child-bearers with different anatomy.
So a beta x omega and an alpha x omega can reproduce. Likely an omega x omega pairing would also reproduce unless they're shooting blanks. That's more up to the author.
Alpha x beta has no partner with the anatomy to bear a child in A/B/O unless the author decides to alter the universe.
Tbh, we have intersex people in the real world, I would imaging in an A/B/O universe there would be people with two genders (alpha+omega, omega+beta, beta+alpha) which might allow either an alpha or a beta to be impregnated.
So BETA x OMEGA can still have a child
ALPHA x BETA can't?I mean If the beta was the uke