A Chestburster is the immature infant form of the Xenomorph which emerges forcibly from the chest of its host, killing it. The chestburster is similar in form to the fully grown alien but far smaller and pale in color.
An egg is laid in the pharynx by a facehugger (young PredQueens are known to insert embryos directly into a host without a Facehugger). It's expeculated that it develops in a manner similar to that of the immature heartworm (Dirofilaria immitis) that migrates through the body absorbing nutrients before establishing itself. Far from the pharynx it's unknown if the embryo travels to the host 's stomach or to the bronchial tubes. Another theory says that the alien begins his life in the host body not as an embryo but as a tumor that later takes the shape of an immature alien. In this theory, what the Facehugger injects in the host is not an infant Xenomorph but a small egg that inserts its proteins in the host DNA and commands its cells to "create" the chestburster wich has both the Alien proteins and the host's.
When fully developed, the creature releases enzymes that softens the bone and surrounding tissues and starts chewing the sternum. This causes the victim great pain, shock, and blood loss. Then, true to its namesake, it pops out. If far away from the Hive it cannot depend on the adults so it actively searches for a nursery, chicken coop, or a food locker, something with a large amount of organic or inorganic matter (such as metals) wich it softens with its acid fluids and directly converts into body mass. It also will attack and kill humans and feed on them. Especially after emerging. They will eat anything of of the person face, genitals, fingers but primarily the organs (brain, heart, lungs, liver, kidneys) Then after a few hours to a few days, it turns into an adult Alien.
An embryo implanted in a pregnant woman will feed directly on the baby and/or placental fluids, and the chestburster will erupt from the stomach of the woman. They often raise themselves to a foot high and are 36 inches long.
The host's characteristics determine the chestburster's features. The Predalien chestburster, for example, sports the signature Yautja mandibles (and grows the dreadlocks later), while quadrupedal hosts produce quadrupedal chestbursters (whereas chestbursters spawned from humans may not even have forelimbs).