Ultrasonography has advanced obstetric practice by enabling relatively detailed assessment of the fetus in utero, including an accurate estimate of gestational age when performed in the first half of pregnancy . This information is invaluable because most obstetrical management decisions are strongly influenced by consideration of fetal development, which closely correlates with fetal age.
Fetal biometric measurements used to calculate gestational age will be reviewed here. The discussion of dating will be based on time since the last menstrual period (menstrual age), not time since conception (conceptional or embryonic age).
The two clinical methods of gestational age assessment are history, using the date of the last menstrual period (LMP) to calculate the estimated date of delivery ("due date" or EDD), and physical examination. The clinical assessment of gestational age or duration of pregnancy reflects the "menstrual age." In contrast, embryologists always date developmental events from the time of fertilization, which is the "embryonic age."
Naegele's rule — Naegele's rule is the most common method of pregnancy dating. The EDD is calculated by counting back three months from the last menstrual period and adding seven days. As an example, if the last menstrual period is February 20, then the EDD will be November 27. If the last menstrual period is May 28, the EDD will be March 4. This method assumes the patient has a 28-day menstrual cycle with fertilization occurring on day 14.
Inaccuracy occurs because many women do not have regular 28-day cycles due to variability in the length of the follicular phase and thus they do not conceive on day 14, while many others are not certain of the date of their last period [2-4]. In addition, early pregnancy bleeding or recent use of hormonal contraceptives may lead to an incorrect assumption of the date of ovulation.