What is Mpreg?
Mpreg stands for male pregnancy, and it originated as a subgenre of gay romance where men can become pregnant. Broadly speaking, there are two main categories within mpreg: shifter and non-shifter. The latter refers to Omegaverse since they are essentially human with shifter traits and hierarchy.
In all Omegaverse, there are alphas, betas, and omegas. The alphas are the top dogs as their name implies, the alpha men and women of society. Betas are the middle class worker bees that keep everything running and are the foundation of the system even though they're often overlooked. The Omega are the nurturing designation who can become bonded for life with their Alphas after receiving an Alpha's bite aka mark. In some versions of Omegaverse, Omegas, regardless of their gender can become pregnant.
How omegas can become pregnant differs per book and author.
In my world a complex miracle of anatomy, male Omegas not only have everything needed for reproduction as a man but were also blessed with a womb to carry their own child. The entire purpose of an Omega was to ensure their world continues to exist by giving birth to other Alphas and Omegas.
Unlike female Omegas, who have two entrances, males only have one. The way I find it easiest to understand is to think of your throat and mouth. With that one entrance you can breathe, eat, or drink and not worry about choking. Your throat has a valve that flips open or closed depending on the need. Likewise, male Omegas have one entrance. Angle down, and you’d end up in the anal tract. Aiming upward, you would find the ‘vaginal’ opening to their reproductive organs.
Just like female Omegas, male Omegas produced slick, a natural lubricant that allows them to take whatever their Alphas has to give them, including their knots without much foreplay. If the slick was flowing, then the Omega was more than ready for sex.
In a lot of cases, it’s not really explained biologically, but operates on the “suspension of disbelief” in the reader. If readers can believe vampires and werewolves exist, why not male pregnancies?
One crucial aspect, however, is the concept of a “heat”. Every omega goes through a heat where they are fertile and desperate for sex. This triggers another phenomenon many Omegaverse readers love: knotting.
Knotting is when the alpha’s dick swells after sex into a knot and stays that way for a while (minutes, usually, sometimes longer) to ensure pregnancy in the omega.
How omegas deliver their babies varies per book/author as well. Since I don't write pregnancy often and I don't plan on writing a delivery on page, so feel free to imagine what makes the most sense to you.
[Portions of this information was gleaned from a blog post by MM/gay romance author Nora Phoenix]