Due Date by Conception Date Calculator
Calculate estimated due date from conception or ovulation date. Compare conception-based dating (266 days) vs LMP method for accurate pregnancy timeline.
- ACOG data · June 2026
- Edited by Martín Rodríguez
- Private — runs on your device
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How to use this calculator
The main result first. The explanation you need, right after.
When to use this calculator
- Verify due date when conception date is known from fertility treatment or confirmed ovulation
- Compare conception-based EDD with LMP-based dating to identify discrepancies
- Track current gestational age in weeks and days from conception
- Plan prenatal screening and delivery logistics with precise dating
- Clarify dating discrepancies between cycle-based and ultrasound estimates
- Coordinate timing for induction or planned cesarean delivery
Conception-Based vs. LMP-Based Pregnancy Dating: Key Parameters
| Parameter | Conception-Based Dating | LMP-Based Dating |
|---|---|---|
| Starting point | Confirmed ovulation / fertilization date | First day of last menstrual period |
| Pregnancy duration | 266 days (38 weeks) | 280 days (40 weeks) |
| Offset between methods | — (reference) | +14 days vs. conception date |
| Accuracy (when method is ideal) | ±3–5 days | ±7–10 days |
| Normal delivery range | 37–42 weeks from conception | 37–42 weeks from LMP |
| Typical use case | IUI/IVF, confirmed OPK/BBT/ultrasound tracking | Routine cycles, initial prenatal screening |
| When less reliable | Estimated conception without documented ovulation | Irregular or anovulatory cycles |
Fuente: American College of Obstetricians and Gynecologists – ACOG Obstetric Care Consensus: Definitions of Term Pregnancy (2013); NICHD – Calculation of Gestational Age (2024)
How it works
What is conception date pregnancy dating?
Conception date pregnancy dating calculates expected delivery by adding 266 days (38 weeks) to the confirmed ovulation or conception date. This method is more accurate than LMP-only dating when ovulation timing is verified through ultrasound or fertility tracking, providing a precise estimated due date.
How It Works
Pregnancy duration from conception to delivery is approximately 266 days (38 weeks), also called the gestational age. This differs from LMP-based pregnancy which counts from the first day of the last menstrual period (about 280 days or 40 weeks) because ovulation typically occurs 14 days after LMP.
Formula
Estimated Due Date (EDD) = Conception Date + 266 days
Gestational Age (weeks + days) = (Today's Date − Conception Date) ÷ 7
Days Until Due Date = EDD − Today's DateWorked Example
Scenario: Woman had confirmed ovulation on May 15, 2026 (verified by OPK or ultrasound follicle tracking).
Key Differences: Conception vs. LMP Dating
| Aspect | Conception-Based | LMP-Based |
|---|---|---|
| Starting Point | Actual fertilization date | First day of last period |
| Duration | 266 days (38 weeks) | 280 days (40 weeks) |
| Accuracy | ±3–5 days when ovulation confirmed | ±7–10 days (regular cycles assumed) |
| When Used | Fertility treatment, confirmed tracking | Routine cycles, initial screening |
When Conception-Based Dating Is More Accurate
✓ Confirmed ovulation via LH surge (OPK), basal body temperature (BBT), or transvaginal ultrasound follicle tracking
✓ Fertility clinic cycles (IUI, IVF) with known fertilization/transfer date
✓ Irregular or anovulatory cycles where LMP dating is unreliable
✓ Large discrepancies between LMP dating and early ultrasound findings
When to Use Caution
✗ Estimated conception date without documented ovulation confirmation (use LMP method or ultrasound instead)
✗ Pregnancy occurring despite hormonal contraception (dating may be uncertain; ultrasound confirm)
✗ Presumed intercourse date without LH/BBT/imaging confirmation (ovulation can occur ±2 days from prediction)
✗ Multifetal pregnancy (multiple gestations date differently; ultrasound-guided dating required)
Important Notes
Example: Confirmed ovulation on May 15, 2026
Frequently asked questions
Is conception date more accurate than LMP for due date calculation?
What's the difference between conception date and ovulation date?
My LMP-based due date doesn't match this conception-based calculation. Why?
How do I know my conception date if I'm not sure?
Is 266 days the same as 38 weeks?
What if I used fertility treatment (IVF/IUI)?
Can I use this calculator if my periods are irregular?
What gestational age should I use for prenatal screening appointments?
Can my due date change after the first ultrasound?
What if I deliver before my due date—is that early or on time?
Sources & references
- American College of Obstetricians and Gynecologists (ACOG) - Obstetric Care Consensus: Definitions of Term Pregnancy — ACOG (2013)
- CDC - Pregnancy and Childbirth: Prenatal Care — Centers for Disease Control and Prevention (2026)
- National Institutes of Health - Calculation of Gestational Age — NICHD (2024)
- WHO - Working Group on Gestational Age and Preterm Birth — World Health Organization (2019)
Methodology & trust
familia calculator reviewed by Martín Rodríguez, editor of Hacé Cuentas, checked against American College of Obstetricians and Gynecologists (ACOG) - Obstetric Care Consensus: Definitions of Term Pregnancy, following our editorial policy and methodology.
Last reviewed: June 2026. Parameters are verified periodically against the cited sources.
Calculations run 100% in your browser. We do not store or transmit your data.
Indicative results. For critical decisions, consult a professional.
Rodríguez, M. (2026). Due Date by Conception Date Calculator. Hacé Cuentas. https://hacecuentas.com/en/due-date-from-conception-date
Content licensed under CC-BY 4.0 — reuse it citing the source with a link to Hacé Cuentas.