An infant born 38 to 42 weeks after conception is considered to be "full term". This is a pretty wide window and allows not only for developmental needs but for some degree of inaccuracy in dating of the last menstrual period and normal variations in the timing of any individual woman's time of ovulation. How many weeks "old" an infant is may be determined purely by dates of the last menstrual period, by sonograms during the pregnancy and by a clinical evaluation of the infant after birth.
This wide window is one reason socially induced labor is not a good idea. A baby with dates of 38 weeks who is delivered after a social induction may developmentally turn out to be 36/37 weeks and suffer some of the problems commonly seen in a near term infant.