Post-term: Overly mature, overly developed, especially as regards a baby that has not been born until well after the usual term pregnancy.
A post-term baby is one born 2 weeks (14 days) or more after the usual 9 months (280 days) of gestation. The gestation (length of the pregnancy) is calculated from the date of the last menstrual period (LMP). This is an important calculation since, if delivery is delayed 3 weeks beyond term, the infant mortality rate skyrockets to 3 times normal.
The definition of a post-term infant has evolved. A few sources still consider a baby post-term if it is born a week after the usual 9 months' gestation but the current trend is to consider a baby post-term only if it is born 2 weeks (14 days) or more after the usual full-term 9 months of gestation.
The terms "post-term" and "postmature" are interchangeable.