It's speculated the host's body begins to grow the embryo due to the Xenomorph's altering of the host's DNA. The embryo grows in the chest cavity just behind the host's sternum for protection, but unfortunately this means a very painful death for the host. Speculation also suggest the embryo uses some of the host's ingested nutrients causing the host to feel hungry upon regaining consciousness. The embryo introduces a chemical into the host's digestive system to keep other Xenomorphs from damaging the host. It's also known that a host will experience a slight fever after removal of a facehugger as their immune systems reacts to the foreign tissue but, due to the altering of the host's DNA, the embryo is able to develop unharmed. The time it takes for an embryo to develop is seventeen to twenty-four hours, with a Queen embryo taking longer. If a host dies with an embryo inside, it doesn't necessarily mean the death of a host. There's no evidence to suggest that the embryo is tied into the host's main systems. The life span of an embryo has yet to be determined.
When the embryo has completed it's period of development inside of the host, it emerges into the next stage in it's life. The burster has built up enough strength into order to tear out of the body cavity inside of the host. The host experiences a great deal of pain during this procedure, resulting in convulsions and loss of all rational thought. This process usually results in the death of the host, which in all practical means isn't all the greatest of way to ensure the reproduction of the species. It would be more efficient if the host were reusable for many aliens, but due to the aliens violent nature, it is only fitting for it to cause death upon birth.
The chestburster punches through the host's sternum sending blood and fragments of bone everywhere killing its host. It's long and slender with small malproportioned limbs and a yet undeveloped head. The chestburster moves by using its tail to push it along much like a serpent. One of the first things a chestburster will do is make its first use of its lungs by letting out a scream or cry. After it does this it will leave its host body. Strangely enough, the chestburster does not feed off the dead host body, why this is has yet to be determined.
Once free of the host, the chestburster is independent to live on it's own. About two feet long from head to tail, it can emerge at different stages of development depending on host conditions, environment, and strength. What the chestburster eats while maturing is also unknown, but it is known that the Xenomorph will molt its shell at least once, possibly twice before final maturation to adulthood. Its metabolism is higher than any of its kin, and it may only take two to three hours to reach its full size of an estimated two meters.