Izuku Midoriya was originally born as an alpha.

However,since he's been dating Katsuki,he noticed some traits between them. That no matter what, Katsuki was more dominant than Izuku. There was always some..domestic behaviour between them.

And the realization hit Izuku once he noticed changes in his own behaviour, of his body, more plump and soft..

Katsuki bitched him. Well,unawarely.

In other words, Izuku was slowly but surely becoming an Omega.