think thats just enigma, alphas (male and female) cant make a beta male pregnant. beta females can get pregnant
Honestly it's up to the authors if they want to make alpha can impregnate beta or not
wait almost forgot, theres a trope where authors will make a beta turn into an omega (whether temporarily or permanently) in which case they can get pregnant by alphas. dogs mask is one example of an alpha temporarily becoming an omega, cheat day is a beta becoming an omega permanently
Why can't a beta get preg tho? Alpha should be able to impregnate anyone tho