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Due Date by Conception Date Calculator

Q: Is conception date more accurate than LMP for due date calculation?

Yes, **when ovulation timing is confirmed** (via LH tracking, BBT, ultrasound, or fertility clinic records). Conception-based dating is accurate to ±3–5 days, while LMP-only dating can vary ±7–10 days depending on cycle regularity and ovulation timing.

Q: What's the difference between conception date and ovulation date?

**Ovulation date** is when the egg is released from the ovary. **Conception date** is when sperm meets egg (fertilization), which typically occurs **within 12–24 hours after ovulation**. Most fertility apps and clinics treat these as the same date for dating purposes.

Q: How do I know my conception date if I'm not sure?

Use **LMP-based dating or ultrasound dating** instead. Conception is uncertain unless you have one of: (1) confirmed ovulation (LH surge, BBT, app), (2) known IUI/IVF procedure date, (3) documented intercourse date + ovulation confirmation, or (4) early ultrasound showing gestational sac diameter.

Q: Is 266 days the same as 38 weeks?

Yes. **266 days ÷ 7 = 38 weeks.** This is the **gestational age from conception** and is 2 weeks shorter than the 40-week LMP count (which includes ~2 weeks from LMP to ovulation).

Q: What if I used fertility treatment (IVF/IUI)?

For **IVF:** Use the **embryo transfer date** (day 3 or day 5 transfer). For **IUI:** Use the **insemination date** (ovulation confirmed shortly after). Your fertility clinic will provide the official dating; this calculator helps you verify or track progress.

Q: Can I use this calculator if my periods are irregular?

Yes, **if you have a confirmed conception/ovulation date.** Irregular cycles make LMP dating unreliable, but ovulation tracking (if done) is just as valid. If ovulation is not confirmed, ask your provider for early ultrasound dating instead.

Q: What gestational age should I use for prenatal screening appointments?

Use **conception-based gestational age** if ovulation is confirmed and dated. If LMP dating and conception dating differ by >5 days, **early ultrasound (before 14 weeks) is the tiebreaker.** Always use the dating your OB/GYN has recorded in your chart.

Q: Can my due date change after the first ultrasound?

Yes. Ultrasound dating in the **first trimester (before 14 weeks) is accurate to ±3–5 days** and is considered the gold standard. If your ultrasound dating differs from conception dating by >5 days, your provider may adjust your official EDD. Dating changes are rare after 14 weeks.

Q: What if I deliver before my due date—is that early or on time?

Delivery timing is classified by **gestational age at birth**, not calendar due date. **Preterm:** 42 weeks. Your accurate dating is crucial for these classifications.

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Data updated: 28 abr 2026 · Source: American College of Obstetricians and Gynecologists (ACOG) - Obstetric Care Consensus: Definitions of Term Pregnancy

Reviewed by: Hacé Cuentas editorial team (política editorial ) · Last reviewed: 28 abr 2026

Was this calculator helpful?

Knowing your conception date or ovulation date provides a more precise estimate of your due date than LMP (last menstrual period) alone. This calculator adds 266 days (38 weeks) to your conception date to estimate your expected delivery date (EDD). Use this when you have confirmed ovulation timing from tracking, ultrasound dating, or fertility clinic records. The result helps your healthcare provider coordinate prenatal care milestones.

Last reviewed: April 28, 2026 Verified by Hacé Cuentas Team Source: American College of Obstetricians and Gynecologists (ACOG) - Obstetric Care Consensus: Definitions of Term Pregnancy, CDC - Pregnancy and Childbirth: Prenatal Care, National Institutes of Health - Calculation of Gestational Age, WHO - Working Group on Gestational Age and Preterm Birth 100% private

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

How it works

2 min read

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 Date

Worked Example

Scenario: Woman had confirmed ovulation on May 15, 2026 (verified by OPK or ultrasound follicle tracking).

Conception Date: May 15, 2026

Add 266 days: February 8, 2027

Estimated Due Date: February 8, 2027

If today is July 1, 2026: 47 days elapsed = 6 weeks + 5 days gestational age

Days until due date: ~223 days remaining

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

Due dates are estimates only. Normal pregnancy range is 37–42 weeks; only ~5% deliver on the exact due date.

Early ultrasound (8–14 weeks) is the gold standard for dating and should be used to confirm or adjust EDD.

Ovulation timing. Even with ovulation tracking, conception can occur within a 12–24 hour window after LH surge.

Healthcare provider input. Always confirm dating with your OB/GYN, especially if conception date conflicts with LMP or ultrasound.

Medical induction. Guidelines typically allow induction at 39 weeks (full term) for accuracy; earlier induction requires specific medical indication.

Frequently asked questions

Is conception date more accurate than LMP for due date calculation?

Yes, when ovulation timing is confirmed (via LH tracking, BBT, ultrasound, or fertility clinic records). Conception-based dating is accurate to ±3–5 days, while LMP-only dating can vary ±7–10 days depending on cycle regularity and ovulation timing.

What's the difference between conception date and ovulation date?

Ovulation date is when the egg is released from the ovary. Conception date is when sperm meets egg (fertilization), which typically occurs within 12–24 hours after ovulation. Most fertility apps and clinics treat these as the same date for dating purposes.

My LMP-based due date doesn't match this conception-based calculation. Why?

If your conception date is confirmed and differs significantly from LMP dating, it usually means ovulation occurred earlier or later than day 14 of your cycle (typical assumption). An early ultrasound (8–14 weeks) will clarify which dating is correct. Always defer to your healthcare provider's official EDD.

How do I know my conception date if I'm not sure?

Use LMP-based dating or ultrasound dating instead. Conception is uncertain unless you have one of: (1) confirmed ovulation (LH surge, BBT, app), (2) known IUI/IVF procedure date, (3) documented intercourse date + ovulation confirmation, or (4) early ultrasound showing gestational sac diameter.

Is 266 days the same as 38 weeks?

Yes. 266 days ÷ 7 = 38 weeks. This is the gestational age from conception and is 2 weeks shorter than the 40-week LMP count (which includes ~2 weeks from LMP to ovulation).

What if I used fertility treatment (IVF/IUI)?

For IVF: Use the embryo transfer date (day 3 or day 5 transfer). For IUI: Use the insemination date (ovulation confirmed shortly after). Your fertility clinic will provide the official dating; this calculator helps you verify or track progress.

Can I use this calculator if my periods are irregular?

Yes, if you have a confirmed conception/ovulation date. Irregular cycles make LMP dating unreliable, but ovulation tracking (if done) is just as valid. If ovulation is not confirmed, ask your provider for early ultrasound dating instead.

What gestational age should I use for prenatal screening appointments?

Use conception-based gestational age if ovulation is confirmed and dated. If LMP dating and conception dating differ by >5 days, early ultrasound (before 14 weeks) is the tiebreaker. Always use the dating your OB/GYN has recorded in your chart.

Can my due date change after the first ultrasound?

Yes. Ultrasound dating in the first trimester (before 14 weeks) is accurate to ±3–5 days and is considered the gold standard. If your ultrasound dating differs from conception dating by >5 days, your provider may adjust your official EDD. Dating changes are rare after 14 weeks.

What if I deliver before my due date—is that early or on time?

Delivery timing is classified by gestational age at birth, not calendar due date. Preterm: <37 weeks. Early term: 37–38 weeks (less ideal). Full term: 39–40 weeks. Late term: 41 weeks. Postterm: >42 weeks. Your accurate dating is crucial for these classifications.