Gestational age is determined in the fetus by menstrual history or ultrasound dating, and in the newborn by physical examination and neuromuscular assessment. This determination is used to assess intrauterine growth, make decisions about the timing of delivery in complicated pregnancies, and predict the infant's clinical course. The most accurate estimation of gestational age is achieved by considering prenatal date (ie, last menstrual period [LMP] and early ultrasound [if available]), in combination with physical and neurological features of the newborn.
●Gestational age (also called menstrual age or menstrual dating) is estimated in completed weeks from the date of the mother's last menstrual period (LMP).
●Estimated date of a term delivery (also known as estimated date of confinement [EDC]) is calculated by adding one year and seven days to the LMP and then subtracting three months. This calculation is known as Naegele's rule and assumes that ovulation occurs two weeks after the LMP.
●Ovulatory or fertilization age of the fetus is the presumed date of ovulation, and is two weeks less than gestational age measured from the LMP. Ovulatory age of the fetus is frequently used by embryologists.
LIMITATIONS OF ASSESSMENT
Accurate menstrual dating depends upon certain maternal knowledge of LMP and regular 28-day cycle length, and is considered to have an accuracy of ±2 weeks.