From what I've seen:
-Batman vs Alien. Batman and a gang of mercenaries encounter a fellow mercenary who has been incubated with an alien. One of the mercenaries shoots him dead, and the alien does not hatch.
-Alien vs Predator (one of the many books, forgot which one). The following lines are mentioned: "Connective tissues to the bloodstream have been dissolved. The creature has stopped metabolizing the host's body fat."
-Some other alien book: Host dummies were being created, bloodstream, body fat, to simulate an incubating environment for an alien.

Here's what I think:

-First and foremost, given the fact that the face hugger injects its stuff into its host, it goes into his stomach. The acid provides an excellent environment for the alien, and it probably explains why the Xenomorph has acid in its blood by nature.
-Like any fertilized egg, a grouping occurs in which a bunch of cells combine to form an embryo. This embryo also forms a placenta-like interface with the host blood so that it can obtain nutrients, mostly fatty acids from the bloodstream. This would explain why the alien would metabolize the fatty tissue from the host.
-If for any reason the host bloodstream is stopped, the alien starves to death as it is unable to obtain nutrients from the host.
-Once the alien attains full growth, it discards the interface and bursts through the host. Note that it either bursts directly from the stomach, or it rises up to the esophagus, where presumably a weaker point is found in which to "burst".

Am I too far off?