
if you haven't figured it out already
when an alpha bites an omega's neck they becomes mates and the omega is unable to have sex with any one else or it'll take a toll on their body. the reason why the omega in the story you read got pregnant was because in was heat, that is be the time that they're most fertile and ready to mate
if you wanted to have more details on why it happens then this should be helpful http://puppylover857.deviantart.com/journal/Omegaverse-Explanation-and-Guide-441250200

from my experience of reading many of these Alpha/Omega stories. It seems to me that Alphas tend to restrain themselves from biting an Omega's neck is because once its bitten it means they belong to the person thats bitten them, they wouldn't be able to have sex with another.
An Omega can't get pregnant, unless they are in heat and the Alpha shoot sperm in their ass.
Thats what i've heard I'm not too sure though. Sorry if I'm wrong.

That's not what causes him to get pregnant. If Yuuri is an omega, then I has a chance of getting pregnant if he doesn't have some sort of conreceptive just like a woman would. The bite is just a make of possession and alpha bestows upon an omega. The bite represents their bond. Once their bonded, they can't have sex with anybody else. It depends on the authors interpretation of the omega verse au, though biting an omega their are attracted to is compulsive during sex. That's with Viktor bit his arm. He didn't have Yuuri's consent, nor did he really know him, even if his alpha wanted to bite him.
Can someone explain me why in a lot of alpha and Omega yaois, when the alpha retains itself from biting it's neck
I mean I saw in another manga(don't remember the name) when the alpha bits the omega, he becomes pregnant
I want to Know why that happened