I know there have been many theories involving viral, bacterial and cancerous growths, all of which can explain why the chestburster is sitting in the chest cavity, because it is grown there.
The problem with the embryo theory is that as a solid mass that is deposited into the host's throat, it has no where to go but to the stomach. I am not the best when it comes to biology but the esophagus doesn't have anything leads into the chest, unless I am missing something.