Standard Rule assumes fertilization occurs on day 14. Adjust cycle length for enhanced precision.
Guidelines & Evidence
Clinical Details
Section 1
When to Use
When to Use
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.
Inclusion Criteria
Apply these rules to all singleton pregnancies. For pregnancies resulting from Assisted Reproductive Technology (ART), the embryo transfer date or oocyte retrieval date provides the most accurate EDD.
Section 2
Formula & Logic
Naegele’s Rule (LNMP-based)
EDD = LNMP + 7 days - 3 months + 1 year
ACOG Re-dating Thresholds
≤ 8 6/7 weeks GA
Re-date if US/LNMP diff > 5 days
9 0/7 to 15 6/7 weeks
Re-date if US/LNMP diff > 7 days
16 0/7 to 21 6/7 weeks
Re-date if US/LNMP diff > 10 days
22 0/7 to 27 6/7 weeks
Re-date if US/LNMP diff > 14 days
≥ 28 0/7 weeks GA
Re-date if US/LNMP diff > 21 days
US Measurement Standards
< 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).
Section 3
Pearls/Pitfalls
The "Gold Standard" Rule
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.
Common Pitfalls
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.
ART Specifics
For IVF, the "LNMP" is calculated as: (Date of Transfer) - (Age of Embryo, e.g., 3 or 5 days) - 14 days. This date is immutable and should not be changed by subsequent US measurements.
Section 4
Next Steps
Setting the Clinical Path
01
Document the Final EDD clearly in the EMR.
02
Schedule anatomy scan at 18–22 weeks based on the established EDD.
03
Calculate timing for GDM screening (24–28 weeks) and GBS screening (36 0/7–37 6/7 weeks).
04
Determine the "41-week" mark for post-dates surveillance (NST/BPP).
Pregnancy Milestone Timetable
11–13+6 Weeks
Nuchal Translucency / NIPT Screening
18–22 Weeks
Anatomy Ultrasound / Cervical Length
24–28 Weeks
Glucose Challenge Test (GDM Screening)
28 Weeks
Rhogam Administration (if Rh negative)
36–37 Weeks
Group B Strep (GBS) Swab
Related Calculators
LNMP to EDD Converter
IVF Dating Tool
Fetal Growth Percentile Chart
Section 5
Evidence Appraisal
Standard Guidelines
Methods for Estimating the Due Date. Committee Opinion No. 700.
ACOG et al. • Obstetrics & Gynecology. 2017;Reaffirmed 2023. The definitive consensus statement on gestational dating and re-dating thresholds.
Accuracy of Ultrasound
Determination of Gestational Age by Ultrasound.
Butt K et al. • J Obstet Gynaecol Can.. 2014;Demonstrates that CRL measurement in the 1st trimester has an accuracy of +/- 3 to 5 days.
Section 6
Literature
Naegele’s History
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.
The Move to Ultrasound
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.
