Chestburster

After implantation, facehuggers die and the embryo's host wakes up afterward, showing no considerable outward negative symptoms and a degree of amnesia regarding events at the time of implantation. Symptoms build acutely after detachment of the facehugger, the most common being sore throat, slight nausea, increased congestion, and moderate to extreme hunger. In later stages where the incubation period is extended in preparation of a queen birth, symptoms will include a shortness of breath, exhaustion, and hemorrhaging (detectable through biological scanners and present in nosebleeds or other seemingly random bleeding incidents), as well as chest pains, caused by lack of chest space due to the chestburster's presence or even premature attempts to escape the host. The incubating embryo takes on some of the host's DNA or traits, such as bipedalism, quadrupedalism, possessing the mandibles of a Predator, and other structural changes. According to Weyland-Yutani medical scientists in Aliens: Colonial Marines, the chestburster will draw nutrients from the host's body in order to develop a placenta as it grows, attaching itself to several major organs in the process. The placenta has cancerous qualities, such that even if the embryo were removed surgically, the placenta would simply cause the affected organs to shut down, resulting in death; the only exceptions to this are from human/xenomorph hybrid hosts like the cloned Ripley 8, who survived an extraction procedure without issue.

Over the course of one to 24 hours—indeterminable in some cases, and sometimes up to a week, in the case of some queens—the embryo develops into a chestburster, at which point, it emerges, violently and fatally ripping open the chest of the host.

There is no on-screen explanation of the reasons for the different incubation times. Fully-grown aliens may avoid harming species acting as hosts for un-emerged chestbursters, though this may only be in the case of a queen embryo.

When a chestburster erupts from the body of a human host, it is less than 30 centimetres (0.98 ft) tall, although the embryo can vary in size from a guinea pig to a large dog depending on the size and species of the host. Its appearance and adaptive characteristics are also determined by the host. Typically, its first instinct upon emerging is to flee and hide until full maturation, as well as find a source of nutrition. However, it soon undergoes a dramatic growth spurt, reaching adult size in a matter of hours; in Alien, the chestburster had grown to 2 metres (6.6 ft) in height by the time the Nostromo crew located it again. The chestburster is shown to have molted before reaching maturity. In Aliens vs. Predator: Requiem, Alien warriors are shown who are still growing, showing shedding skin. In the unrated cut, the Predalien is shown wiping off its final molted skin at the film's start.

The chestburster was designed by Alien director Ridley Scott and constructed by special effects artist Roger Dicken. Giger had produced a model of a chestburster that resembled a "degenerate plucked turkey" and was far too large to fit inside a ribcage. Much to Giger's dismay, his model reduced the production team to fits of laughter on sight. Scott drafted a series of alternative designs for the chestburster based on the philosophy of working "back [from the adult] to the child" and ultimately produced "something phallic". The chestburster in the original Alien was armless, but arms were added in Aliens to facilitate the creature crawling its way out of its host's corpse. This concept would be abandoned in Alien: Resurrection, but it would return in Alien: Covenant.