Naegele’s Rule: EDD = LMP + 1 year − 3 months + 7 days
Conception: EDD = Conception + 266 days
Due Date Calculator 2025: LMP, Conception, Ultrasound — Full Accuracy Guide
Due date (EDD) is 40 weeks from LMP. Our calculator uses **Naegele’s rule**, **conception**, or **ultrasound** for **±3 day accuracy** in first trimester.
Calculation Methods Compared
| Method | Formula | Best For | Accuracy |
|---|---|---|---|
| LMP | LMP + 280 days | Regular cycles | ±7 days |
| Conception | Conception + 266 days | IVF or known date | ±3 days |
| Ultrasound | US Date + (280 − GA) | Irregular cycles | ±3–5 days (1st tri) |
Trimester Breakdown
- 1st: 0–12 weeks (organ formation)
- 2nd: 13–27 weeks (growth, movement)
- 3rd: 28–40 weeks (lung maturity)
Why Due Dates Change
- Early ultrasound overrides LMP
- Multiples: often deliver 37–38 weeks
- Growth scans adjust EDD after 20 weeks
Full-Term Redefined (ACOG)
- Early Term: 37w0d – 38w6d
- Full Term: 39w0d – 40w6d
- Late Term: 41w0d – 41w6d
- Post-Term: ≥42w0d
Irregular Cycles?
Use **conception date** or **early ultrasound**. LMP assumes 28-day cycle.
IVF Pregnancies
EDD = **Embryo transfer + 263 days** (5-day blastocyst) or **+266 days** (3-day).
Limitations
- Only 4% deliver on EDD
- Not for twins/triplets
- Does not predict labor
Pro Tips
- Get **8–11 week ultrasound** for best dating
- Track with Gestational Age
- Plan C-section after 39 weeks
FAQs
Can I induce at 39 weeks? Yes, if no complications.
What if I’m overdue? Monitor after 41 weeks.
Is 37 weeks safe? Yes — early term.
Conclusion
Use our **2025 Due Date Calculator** with LMP, conception, or ultrasound. Always confirm with OB/GYN.
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