• Establishing the Estimated Date of Delivery (EDD) at the initial prenatal visit.
• Reconciling discrepancies between Last Normal Menstrual Period (LNMP) and ultrasound (US) measurements.
• Confirming gestational age (GA) before elective inductions or Cesarean sections.
• Assessing fetal growth velocity in later pregnancy based on an established "Gold Standard" early scan.

• < 14 0/7 weeks: Crown-Rump Length (CRL) is the most accurate parameter.
• ≥ 14 0/7 weeks: Composite of Biparietal Diameter (BPD), Head Circumference (HC), Abdominal Circumference (AC), and Femur Length (FL).

• The earliest ultrasound with a CRL is the most accurate and should be used to set the EDD for the remainder of the pregnancy.
• If multiple early scans exist, use the very first one performed (ideally between 7–12 weeks).
• Do NOT re-date a pregnancy in the third trimester based on an ultrasound if a first-trimester scan was already performed.

• Using LNMP in patients with irregular cycles (PCOS) often leads to significant overestimation of GA.
• Subchorionic hemorrhage can sometimes be mistaken for a gestational sac in extremely early scans.
• Later in pregnancy, fetal growth restriction (FGR) or macrosomia can skew dating; always rely on the earliest available data.

• Document the Final EDD clearly in the EMR.
• Schedule anatomy scan at 18–22 weeks based on the established EDD.
• Calculate timing for GDM screening (24–28 weeks) and GBS screening (36 0/7–37 6/7 weeks).
• Determine the "41-week" mark for post-dates surveillance (NST/BPP).

Franz Karl Naegele (1778–1851), a German obstetrician, popularized the rule for calculating due dates based on the work of Hermann Boerhaave. It assumes a 28-day cycle with ovulation occurring exactly on day 14.

Historically, "Post-term" pregnancy was often a misdiagnosis due to LNMP errors. The shift toward ultrasound-based dating has significantly reduced the rate of unnecessary inductions and improved the management of growth disorders.