TL;DR
- LMP method: EDD = LMP + 280 days (assumes 28-day cycle with ovulation ~day 14). Adjust if your cycle is longer/shorter.
- Conception method: EDD = Conception + 266 days (works well for IVF/IUI or a clearly known date).
- Ultrasound: Early first-trimester CRL dating is typically the most precise; many clinicians will update EDD to match an early scan if it disagrees meaningfully with LMP.
The three dating methods, side by side
| Method | How it’s calculated | Best used when | Limitations |
|---|---|---|---|
| LMP (Naegele’s rule) | LMP + 280 days (40 weeks) | Typical 28-day cycles, reasonably certain LMP | Off if cycles aren’t 28 days or ovulation isn’t ~day 14; recall errors |
| Conception date | Conception + 266 days (38 weeks) | IVF/IUI or well-known conception window | If “date” is just the only intercourse date, you can still be off by several days |
| Ultrasound (CRL early) | Embryo crown–rump length → gestational age | Early scans (≈7–12 weeks) with good visualization | Later scans are less precise; growth variation increases |
Cycle-length adjustments (don’t skip this)
If your cycle isn’t 28 days
Adjusted EDD (from LMP) = LMP + 280 days + (Cycle length − 28)
Example (31-day cycles): add 3 days to the LMP-based EDD. Example (25-day cycles): subtract 3 days.
Reason: ovulation tends to occur ~14 days before your next period. Longer cycles = later ovulation; shorter cycles = earlier ovulation.
IVF specifics (actual formulas)
- Day-5 (blastocyst) transfer: EDD = Transfer date + 261 days (because 266 − 5).
- Day-3 (cleavage) transfer: EDD = Transfer date + 263 days (because 266 − 3).
- Or use embryo age directly: EDD = Transfer date + 266 days − embryo age (in days).
Ultrasound accuracy by trimester (rough ranges)
- First trimester (CRL, ~7–12 weeks): tightest window; often within about a week either way.
- Second trimester: less precise as growth differences emerge; think ~±10–14 days.
- Third trimester: least precise for dating; growth variation dominates.
Bottom line: If an early ultrasound disagrees with LMP beyond a certain threshold, many clinicians update the EDD to the ultrasound date and stick with it.
Which date “wins” in practice?
- Start with LMP (adjust for cycle length) or conception + 266 if you truly know conception.
- Early ultrasound available? That often becomes the official EDD if it conflicts meaningfully with the above.
- Pick one official EDD and keep it. Constantly changing the target creates confusion about growth and timing.
Edge cases you’ll hear about
- Irregular cycles: LMP is shaky. Ultrasound (early) is your best anchor; otherwise use conception/OPK evidence.
- Conception uncertainty: One intercourse day ≠ guaranteed conception that day. Sperm can survive up to ~5 days.
- Multiples (twins, etc.): Dating uses the same methods; many deliver earlier than the “EDD,” but the calculation method itself doesn’t change.
- Late first visit: If your first scan is late second trimester, accept that dating precision is worse; don’t retrofit growth to a shifting EDD.
Reality check: An EDD is an estimate, not a promise. Most births don’t land exactly on it. The job is consistent dating, not perfect prediction.
Quick examples
- Typical cycle, LMP known: LMP = Jan 1 → EDD = Oct 8 (Jan 1 + 280 days).
- Long cycles (32 days): LMP = Jan 1 → Base EDD Oct 8, then +4 days → Oct 12.
- Conception known (IUI on Jan 15): EDD = Jan 15 + 266 days → Oct 8.
- IVF day-5 transfer on Feb 1: EDD = Feb 1 + 261 days → Oct 20.
Related on this site
- Due Date & Ovulation Calculator
- When to test for pregnancy
- Estimating ovulation with irregular cycles
This page is educational only and not medical advice. If your EDD changes or your cycle is highly irregular, bring your data to your clinician and agree on a single, official EDD.