* Stedman’s Online Medical Dictionary, 27th Edition defines “gestational age” as follows: “n. 1. in embryology, the age of a conceptus expressed in elapsed time since conception; 2. in obstetrics, the developmental age of a fetus, usually based on the presumed first day of the last normal menstrual period.”
The postfertilization age (that is, the embryological “gestational age”) is assumed to be approximately two weeks less than the obstetrical “gestational age” (sometimes referred to as “LMP,” meaning since the first day of the “Last Menstrual Period”), since ovulation and consequently fertilization is assumed to take place about midway through the menstrual cycle. Cf. Engle W, American Academy of Pediatrics Committee on Fetus and Newborn (2004). “Age terminology during the perinatal period”. Pediatrics 114 (5): 1362–4.
According the National Institutes of Health, U.S. National Library of Medicine, “Gestation is the period of time between conception and birth, during which the fetus grows and develops inside its mother’s uterus. Gestational age is the time measured from the first day of the woman’s last menstrual cycle to the current date. It is measured in weeks. A pregnancy of normal gestation is approximately 40 weeks, with a normal range of 38 to 42 weeks. Infants born before 37 weeks are considered premature. Infants born after 42 weeks are considered postmature. This gestational maturity rating is measured by the Ballard scale or Dubowitz exam. Strictly speaking, the gestational age of a fetus or infant is a measurement of time inside of the uterus. Gestational age can be determined before the baby is born or at its birth. Prior to birth, growth is determined with ultrasound by measuring the diameter of the head and comparing the head circumference and the abdominal circumference.”