Species: chestburster

A Chestburster is the infant form of the Xenomorph species, which are impregnated into living hosts by the Facehugger. Earning its' namesake, the Chestburster (Canonicity) emerges forcibly from the chest of its host, killing the host. This is usually caused by following:

A Xenomorph doing what xenomorphs do.
Parasites or Parasite
Oral egg insertion
Facehugger

Also See:
Abdominal bulge
Pectoral bulge
Pregnant male
Pregnant female