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. A first trimester ultrasound examination can decrease the rate of induction for postterm pregnancy  and improve estimation of viability when complications occur at the limit of viability. (See "Limit of viability".)
Fetal biometric measurements used to calculate gestational age and estimated date of delivery ("due date" or EDD) will be reviewed here. The discussion of pregnancy dating will be based on time since the last menstrual period (LMP, menstrual age), not time since conception (conceptional or embryonic age).
ESTIMATED DATE OF DELIVERY
The estimated date of delivery (EDD) is 280 days from the onset of the last menstrual period (LMP) and 266 days from date of conception. Although fertilization of an oocyte in vivo cannot be detected by any laboratory or imaging method, it occurs within 24 hours of ovulation, which can be accurately predicted by detecting the luteinizing hormone (LH) surge or by ultrasound examination of the periovulatory follicle (see "Evaluation of the menstrual cycle and timing of ovulation") Only 4 percent of women deliver on their EDD, in part because of the limitations of methods used to estimate gestational age, but also because of natural biologic variation in the pace of fetal maturation and the timing of natural delivery .
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 (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 LMP and adding seven days. As an example, if the LMP is February 20, then the EDD will be November 27. If the LMP is May 28, then the EDD will be March 4. This method assumes the patient has a 28-day menstrual cycle with fertilization occurring on day 14.