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.
- 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.
- Ovaluatory or fertilization age of the fetus is the presumed date of ovulation, and is two weeks less than gestational age measured from the LMP. Ovaluatory 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.
Biological differences in the duration of pregnancy also contribute to errors. In an analysis of pregnancy duration in 9355 mothers, for example, women who were multiparous, younger than 19 or older than 34 years, or black had shorter gestations than did women who were primiparous, aged 19 to 34 years, or white . In another study, the median duration from assumed ovulation to delivery was significantly longer in primaparas than multiparas (274 versus 269 days) and longer than the predicted 266 days . Assuming that the average length of pregnancy is 282 instead of 280 days reduced the average discrepancy between the date of delivery predicted from the LMP or biparietal diameter measured using ultrasound . However, when maternal history is reliable, Naegele's rule remains an accepted standard for calculating gestational age and EDC.
The prenatal assessment of gestational age is discussed separately. (See "Prenatal assessment of gestational age".)