I heard that one of the people involved in one of the Alien films (don't remember who) suggested that the facehugger didn't lay any embryo, instead it altered some of the host's DNA so that the host's body would make the embryo from the virus or whatever that the facehugger gave to the host and then the embryo would chestburst when fully mature. But, I think AVPR pretty much stomped on that theory when you can see egg-like pods moving down the pregnant woman's throat when being eggbarfed upon by Chet. Though, I could probably retcon it by saying that those embryos were from a pre-queen and could be different from the reproductive method of facehuggers.