Coffin birth, known in academia by the more accurate term postmortem fetal extrusion, is the expulsion of a nonviable fetus through the vaginal opening of the decomposing body of a pregnant woman as a result of the increasing pressure of intraabdominal gases. This kind of postmortem delivery occurs very rarely during the decomposition of a body. The practice of chemical preservation, whereby chemical preservatives and disinfectant solutions are pumped into a body to replace natural body fluids (and the bacteria that reside therein), have made the occurrence of "coffin birth" so rare that the topic is rarely mentioned in international medical discourse.
Typically during the decomposition of a human body, naturally occurring bacteria in the organs of the abdominal cavity (such as the stomach and intestines) generate gases as by-products of metabolism, which causes the body to swell. In some cases, the confined pressure of the gases can squeeze the uterus (the womb), even forcing it downward, and it may turn inside-out and be forced out of the body through the vaginal opening (a process called prolapse). If a fetus is contained within the uterus, it could therefore be expelled from the mother's body through the vaginal opening when the uterus turns inside-out, in a process that, to outward appearances, mimics childbirth. The main differences lie in the state of the mother and child (whether alive or dead) and the mechanism of delivery: in the event of natural, live childbirth, the mother's contractions encourage the infant to emerge from the womb; in a case of coffin birth, built-up gas pressure within the putrefied body of a pregnant woman pushes the dead fetus from the body of the mother.
Cases have been recorded by medical authorities since the 16th century, though some archaeological cases provide evidence for its occurrence in many periods of human history. While cases of postmortem fetal expulsion have always been rare, the phenomenon has been recorded under disparate circumstances and is occasionally seen in a modern forensic context when the body of a pregnant woman lies undisturbed and undiscovered for some time following death. There are numerous examples that have demonstrated that the term "coffin birth" is a misnomer under many circumstances.