It's fake that's how
First let's start with a picture of the pelvic bone.
https://en.wikipedia.org/wiki/Pelvis
There is an opening through the pelvic bone. The rectum, urethra, and vagina go down through the hole. The rectum does not go up the outside of the pelvic bone so no baby bump in the back. Women's pelvises have a larger hole than men because we're expected to get a whole baby through there. I would assume that an omega's pelvic bone is shaped more like a woman's than a man's though from drawings in mangas they don't seem to have hips like a woman. As to how an omega actually gives birth, that seems to have been left up to our imagination. Or not as some people are grossed out by that stuff. Hope I helped some.
Most, if not all, omegaverse manga have some sort of explanation at the beginning so I just went and chose random one http://www.mangago.zone/read-manga/sayonara_alpha/ should still explain it though
In We Only Held Hands the men in the family can get pregnant if they sleep while holding hands with another person. After a bit a seed comes out of their navel. They plant the seed and a flower grows and a tiny baby develops in the flower. When the baby is 'ripe' they take it out and start bottle feeding and the baby grows naturally from there. It sounds like the bullshit parents used to tell their kids and sometimes still do on the theory that if they aren't told about sex and how babies are conceived it won't happen. But then they figure something out while fooling around and are kicked out of the house still wondering how they got pregnant. Sorry. The whole no sex ed thing pisses me off.
Mpreg is when a man gets impregnt by another man and bears his children, my question is how? See females have vaginanal intercourse and the womb is connected there. But with all the Mpreg I've been seeing, the men have a baby bump at the same place as females, which means their womb is at the front. But they have intercourse through the ass. And seeing a baby bump on a man's back would definitely hurt their back and look scary. But if the womb is at their front and the release is in the asshole, how did the sperm get from there to the womb? What connects the two? Are they even connected? (men don't actually have a womb inside their bodies naturally in real life but how else do they hold babies inside their body? So I'll assume that they have a womb suddenly) And the fact that later the child will come out. OUT FROM WHERE??? Is it from their ass? A C-section? Magically appearing? From the belly button (which was one of the ways I saw birth from a man, like the manga is here on this site.) I NEED SERIOUS HELP. Sorry if this seemed outta the blue, I've been trying to figure it out for quite some time.