Postmaturity: Overly mature, overly developed, especially as regards a baby.
A baby 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 postmature infant has evolved. A few sources still consider a baby postmature if it is born a week after the usual 9 months' gestation but the current trend is to consider a baby postmature ONLY if it is born 2 weeks (14 days) or more after the usual 9 months of gestation.
The terms "postmaturity" and "post-term" are interchangeable.