As that is, on the face of it, a difficult concept, it is worth quoting the ActPosthumous births if conception after death
Posthumous births if conception after death91.1 (1) A descendant of a deceased person, conceived and born after the person's death, inherits as if the descendant had been born in the lifetime of the deceased person and had survived the deceased person ...
A. Posthumous: adj. 1. Of an action, reputation, etc.: occurring, arising, or continuing after death.
1966 P. Larkin in Crit. Q. 8 174 Hardy's reputation has not taken the accustomed posthumous dip: his books have continued to sell.
A. Postmortem adv. After death.
B. adj. Taking place, formed, or done after death. Also in extended use: after the conclusion of a matter
1996 Times 20 May 3/7 Post-mortem examinations will be held today to discover how they died.
In other words: postmortem = after one's own death; posthumous = after the death of someone else.Coffin birth, known in academia by the more accurate term postmortem fetal extrusion, is the expulsion of a nonviable fetus through the vaginalopening 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. (http://en.wikipedia.org/wiki/Coffin_birth)