MPreg

MPreg is a shortening of the words 'male pregnancy,' and stories marked as MPreg are exactly what that description suggests.

Why Would This Ever Happen?
There are many possible reasons for MPreg to come up in a story, despite the obvious Bad Biology that surrounds even possibly making a male pregnancy a reality. However, they boil down to three main reasons:
 * The story is a horror/somewhat frightening story, containing body horror or at least the extremely uncomfortable situation of a man's body not behaving as it should. (Example, the episode Unexpected of Star Trek: Enterprise, or Geiger's Alien.)
 * The story is a comedy/humorous story, and the absurd situation is played for laughs. (The movie Junior, with Arnold Schwarzenegger. This movie does contain an element of body horror, too, though.)

The first situation is very distressing and scary, often connected with phobias of emasculation, parasites, or the loss of control over one's body. The second can potentially be sort of funny, in a 'see how you like it' sense. The third situation is the one that the PPC most often deals with, often in bad slash.
 * The story is a romance story: a slash fic, often involving yaoi. This may or may not be bad slash... but unfortunately often is.

Why would any writer invoke MPreg as romantic? There are some common reasons:
 * 1) The writer genuinely wants to invoke a story about a couple coping with a very uncomfortable and unlikely situation with a possible happy ending concerning a child at the end (or maybe a sad ending, if the child/'mother' doesn't make it...). This is rare, to say the least.
 * 2) The writer wants the child to be the outcome of a sex scene in the fic, and doesn't care how.
 * 3) The writer wants the child to inherit any powers that the male parents may have.
 * 4) The writer thinks babies are cute and that the apex of every relationship is to have a child to cement twu wub forever.
 * 5) The writer does not think that the characters could ever love an adopted child as much as they would a child of their own flesh and blood, despite many same-sex couples being perfectly happy with the option.

Treating Those Afflicted with MPreg
The Department of Sufficiently Advanced Technology has come up with a remedy for canon-breaking MPreg: the Embryo Extraction Kit. Also used to extract uncanonical offspring of females, the instrument resembles the de-bugger used in The Matrix, but hooks up to a life-supporting receptacle, depositing the embryo into a friendlier environment than some poor canonical's intestinal tract, or Flowers forbid, uncanoncial womb. Since most MPreg children could never exist in canon, the fetuses are under the care of Medical Department until birth, after which they are placed in Canon Nursery School.

Good Mpreg: Possible?
Just like good scriptfic or smutfic, good mpreg fic does exist, but it's much rarer than the bad sort and takes a great deal more skill to write properly. Signs of a good mpreg fic include:
 * Explaining why the male is pregnant, and how this could work. Maybe there's technology or magic that can make it happen. Maybe they're part of a species where males do occasionally get pregnant. Maybe there's shape-shifting involved, or the male in question is transgender and still has the required uterus and baby-making accessories.
 * The story may take place in an AUverse where all males can get pregnant, but this requires the usual careful crafting of the AU itself, taking into account what might be different if this crucial gender difference had never existed.
 * Making sure that the male(s) in question stay in character. Turning one into a hormonal pregnant person and the other into a fluffy, caring nursemaid is right out.
 * Having people react realistically to the problem of the pregnant man. If this takes place in the Real World, for example, most people would assume the man had a tumor or was simply getting fat.
 * Where, exactly, does the baby emerge? Clue: The human anus does not work that way. Well-written mpreg will generally do something like avoid the issue with a C-section or else use magical/technological means to provide female anatomy that can serve the purpose.
 * Could this story be done with an adopted child? If so, there may be no reason to have an mpreg fic at all.
 * If this story doesn't creep or gross most people out (unintentionally), if you don't read it and feel it to be stupid, or if it's really and truly enjoyable, it might just be goodfic.

The Division of Mpreg
The Department of Bad Slash's Division of Mpreg takes care of administering the above cure, and Bad Slash agents in general may often run across and have to fix Mpreg in their own fics.

See also Squick.