Justified: Bob is an alien, a fully-functional hermaphrodite, or of some other male pregnancy -compatible type of being, and is naturally capable of getting pregnant and carrying babies.
Bob is undergoing "sympathetic pregnancy", due to the actually pregnant Alice's pheromones or other physiological cues. He shows symptoms of pregnancy, but is not actually pregnant.
Inverted: Alice gets pregnant in a universe where men getting the babies is the natural way of things.
Alice is impregnated by another woman.
Subverted: Bob chalks up his nausea and slightly swollen belly to last night's taco binge.
Double Subverted: But goes to the doctor when he doesn't get better, only to find that he's not only pregnant, but 3 months along.
Reconstructed: Bob's physiology (either naturally or because A Wizard Did It) averts these problems: apparently, Bob has a fully-functional uterus in there somewhere, and his pelvis will somehow accomodate the birth (or he'll just have a C-section.) And of course, Bob eventually accepts his role as a...parent after the initial shock wears off.
Zig Zagged: Some men (often Uke archetypes) are capable of becoming pregnant, others are not.
Averted: Nothing related to male pregnancy is mentioned.
Invoked: Andy and Bob seek out a wizard, and are given a potion that will allow Bob to carry Andy's child.
Defied: Bob is somehow capable of bearing children, and he and Andy know this. They are Genre Savvy enough to know that male pregnancy could be problematic (see "Deconstructed"), so they use condoms and spermicide for Bob's safety. If they decide later that they want a child, they will see about adopting one.
Or, unable to bear children the natural(?) way, they enlist the help of the local Mad Scientist to genetically engineer a child from their genes, or get the assistance of a doctor and a willing female friend or paid surrogate.
Conversed: Alice "I don't get this whole {MPREG} genre, if they want a baby, why can't they adopt it or something?" Carl "Maybe the author likes the idea of malekind learning of the hardships of female reproduction."
Played For Drama: Bob's pregnancy is difficult, he and Andy face scorn from others, the pregnancy was completely unplanned (and possibly unwanted), and this all takes a toll on Andy and Bob's relationship not to mention other aspects of life such as social status and connections to friends and relatives.
The pregnancy was unwanted, but Bob, remembering he was born from an unwanted pregnancy as well, decides to go along with it despite the risks because he was given this much as well.
