Right, mpreg. It's a speculative fiction trope where a character who is male or masculine-presenting becomes pregnant and gives birth. The mechanics vary wildly by genre—sometimes it's magical, sometimes sci-fi with advanced tech or alien biology, sometimes just a fact of that world's setting. I think the core thematic pull isn't really the 'how' of the biology, but what that situation allows an author to explore.
You get these intense explorations of bodily autonomy, transformation, and vulnerability from a perspective not typically centered in narratives about reproduction. It can deconstruct traditional family structures and gender roles, asking what 'fatherhood' or 'motherhood' means when the biological lines are blurred. In romance, especially omegaverse, it often ties into themes of destiny, bonding, and protective instincts, but it can also be a vehicle for body horror or societal critique in darker fiction.
Some readers are purely in it for the fluffy domestic scenarios or the high-stakes emotional drama of a pregnancy in unconventional circumstances. Others seek out the trope specifically for its subversion of norms. I've seen it handle everything from sweet, slice-of-life family building in contemporary AU fics to grueling survival plots in dystopian settings where the pregnancy is a central point of conflict. The tone really dictates which themes come to the forefront.