Alien chestburster gestates in the chest of victim, and is connected by blood vessels to the host organism, which makes it a parasite.
Interestingly enough, the gestation period varies from a few minutes to a few days. Perhaps it has do to with whether the embryo in question is a common warrior type or a queen. In the latter case gestation seems to last days.
Once the chestburster makes its gory entrance this alien baby grows incredibly fast and reaches the full size in a matter of hours.
The fascinating part is that it reaches its full size without any apparent source of food around. It is yet another indication that the alien is an artificially designed life form.
Essentially, the chestburster is the miniature replica of the adult xenomorph.