Infant, post-term: An overly mature baby that has not been born until well after the usual term pregnancy. A post-term baby is specifically 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 a very 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 infant" and "postmature infant" are interchangeable.