How Naegele's Rule works
Naegele's Rule, developed by Franz Karl Naegele in 1812, remains the standard clinical method for estimating EDD. It assumes an average cycle of 28 days with ovulation on day 14.
Naegele's Rule Formula
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EDD = First Day of LMP + 7 days + 9 months
This is equivalent to: First Day of LMP + 280 days (40 weeks)
This is equivalent to: First Day of LMP + 280 days (40 weeks)
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Example: LMP = January 1, 2025 → Add 7 days = January 8 → Add 9 months = October 8, 2025
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Cycle adjustment: For cycles other than 28 days, the calculator adjusts ovulation timing. For a 32-day cycle, ovulation typically occurs on day 18 (rather than day 14), so the EDD shifts +4 days.
Trimester Timeline
| Trimester | Weeks | Key Milestones |
|---|---|---|
| First Trimester | Weeks 1–13 | Implantation, major organ formation, heartbeat detectable (~week 6–8) |
| Second Trimester | Weeks 14–26 | Anatomy scan (~week 20), movement felt, rapid growth |
| Third Trimester | Weeks 27–40+ | Lung maturation, brain development, preparation for birth |
| Full Term | Weeks 39–40 | Optimal gestational age for birth |
| Late Term / Overdue | Weeks 41–42 | Monitoring increases; induction may be discussed |
FAQ
Studies show only about 4% of women deliver on their exact EDD. However, 70–75% deliver within 10 days either side. The EDD is best understood as the midpoint of a 5-week window (weeks 38–42). First-trimester ultrasound is generally considered more accurate than LMP-based dating when the two disagree by more than 7 days.
For irregular cycles, LMP-based dating is less reliable. The calculator's cycle-length adjustment helps for predictably long or short cycles, but for highly irregular cycles (e.g., PCOS), first-trimester ultrasound dating is more accurate. Discuss dating method with your healthcare provider.
Your doctor may use Naegele's Rule as a starting point, but will typically confirm or revise the EDD using first-trimester ultrasound measurements (crown-rump length is the most accurate dating method before 14 weeks). If the ultrasound EDD differs from the LMP EDD by more than 7 days in the first trimester, providers generally use the ultrasound date.
Medical Disclaimer: This calculator is an educational tool providing estimates based on Naegele's Rule. It is not a substitute for professional prenatal care. Due dates calculated here may differ from those determined by your healthcare provider using ultrasound. See our full Medical Disclaimer.