How does the reverse due date calculator work?

It applies standard obstetric day counts in reverse from your anchor date (EDD or birth date). Conception estimate = anchor − 266 days (38-week fetal-age model). LMP estimate = anchor − (280 + (cycle length − 28)) days. Example EDD Sep 1, 2026, 28-day cycle: conception ≈ Dec 9, 2025; LMP ≈ Nov 25, 2025. Conception stays fixed at 266 days back in this model; cycle length only shifts LMP.

Why is conception 266 days but pregnancy counting uses 280 days?

Clinicians date pregnancy from the first day of the last menstrual period (LMP)—about two weeks before fertilization on a 28-day cycle. So “40 weeks pregnant” at due date equals ~38 weeks from conception. Reverse math: EDD − 266 ≈ conception; EDD − 280 ≈ LMP when cycle is 28 days. That two-week gap is why gestational week 4 on apps is roughly two weeks after conception.

Why does cycle length matter for LMP but not conception in this calculator?

The EDD anchor already embeds when delivery is expected. Conception is modeled as a fixed 266 days before that anchor. Longer cycles (e.g., 35 days) usually mean ovulation later in the cycle, so the period start (LMP) must sit further back: subtract 280 + (35−28) = 287 days from EDD instead of 280. Shorter 24-day cycles subtract 276 days. Ovulation day shifts; the 266-day conception offset from EDD does not change in this teaching model.

Is the calculated conception date exact?

No. Sperm may survive ~3–5 days in fertile cervical mucus; the egg is fertilizable ~12–24 hours after ovulation. Intercourse anywhere in the ~5-day window before the estimated conception day can explain the same pregnancy. This tool shows a central estimate plus a fertile window (conception − 4 days through conception + 1 day)—not a single timestamp of intercourse.

Can I use my baby's birth date instead of the due date?

Yes—enter the actual birth date as the anchor. Only ~4–5% of babies arrive on the original EDD; most deliver within ±2 weeks. Reverse math from birth date yields a different conception estimate than reverse from the prenatal EDD. Example: birth May 10, 2026 (28-day cycle) → conception ≈ Aug 17, 2025 vs EDD-based dates if delivery was early or late.

How accurate is reverse due date vs ultrasound dating?

First-trimester crown–rump length ultrasound is often more accurate than LMP recall—if ultrasound and LMP disagree by >5–7 days, many guidelines use ultrasound for official EDD. Reverse due date from an EDD that was itself ultrasound-set is more reliable than reverse from a guessed LMP. Always prefer documented clinical dates over app estimates.

How do irregular periods affect reverse due date estimates?

Irregular cycles break the assumption that ovulation occurs at a predictable offset from LMP. A single “average” cycle length may not describe the conception cycle. Better approaches: early ultrasound dating, ovulation tracking for the conception cycle, or IVF records. This calculator is best when cycles are fairly regular and you know a reliable EDD.

Can stress or illness change my conception date estimate?

Stress, travel, or illness can delay ovulation in a given cycle—shifting true conception later than day-14 math predicts and potentially shifting true due date later too. If you conceived during a stressed cycle, reverse EDD math may place conception too early. Ultrasound or known ovulation-positive days refine the window.

Is IVF due date calculated the same way?

No. IVF has known fertilization or embryo transfer dates—due dates are calculated forward from transfer day and embryo stage (e.g., 5-day blastocyst adds set offsets). Use our IVF due date calculator instead of 266-day reverse guessing. Fresh vs frozen transfer and lab timing make LMP-based reverse math inappropriate.

What trimester dates does this calculator show?

From estimated LMP: end of 1st trimester ≈ LMP + 91 days (through week 13); end of 2nd trimester ≈ LMP + 189 days (through week 27); EDD ≈ LMP + 280 on a 28-day model. Example LMP Nov 25, 2025 → 1st trimester ends ~Feb 24, 2026; 2nd ~Jun 2, 2026; EDD Sep 1, 2026.

Can I get pregnant right after my period?

Uncommon but possible with short cycles. A 21-day cycle may ovulate ~day 8; sperm living 5 days could bridge late bleeding to ovulation. Reverse due date from a full-term EDD cannot tell which intercourse day conceived—only a plausible fertile window.

How does this relate to the period or ovulation calculators?

Forward tools (period calculator, ovulation window) predict next ovulation from LMP + cycle length. This page runs backward from EDD to infer LMP and conception—useful when you know due date but not period dates. Pair forward and reverse estimates; large gaps suggest irregular ovulation or dating error.

What if my provider changed my due date after ultrasound?

Use the official EDD from your chart (often ultrasound-adjusted) as the anchor—not an early app guess. A shift of 7–10 days changes conception estimate by the same amount (266 days back). Document both original LMP-based and ultrasound EDDs if comparing.

Is this reverse due date calculator medical advice?

No. It is an educational timeline tool using population-average obstetric rules. It does not establish paternity, legal dates, or clinical management. For dating, prenatal care, or fertility questions, consult your obstetric clinician or midwife.

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Obstetrics & maternity

Reverse Due Date Calculator — Conception & LMP From EDD

Count backward from an estimated due date (EDD) or birth date to estimate conception (EDD − 266 days) and last menstrual period with cycle-length adjustment. Pair with our gestational age calculator. Educational—ultrasound dating overrides when it differs.

Last updated: June 5, 2026

Conception date + fertile window (5-day teaching band)

Cycle-adjusted LMP and trimester milestone dates

Comparison tables and step-by-step formula examples

Fertile window vs one conception day

Sperm can survive several days before ovulation. Intercourse in the days leading up to the estimated conception date can still explain the same pregnancy.

Reverse Due Date Calculator

Work backward from EDD or birth date to estimated conception, LMP, and trimester milestones.

Due date or birth date

Average cycle length (days)

Estimated conception

Tuesday, December 9, 2025

Fertile window: Dec 5, 2025 – Dec 10, 2025

Timeline

LMP (est.)Nov 25, 2025

End 1st trimesterFeb 24, 2026

End 2nd trimesterJun 2, 2026

EDD / anchorSep 1, 2026

Clinical estimates using 266- and 280-day rules. Ultrasound dating overrides when records disagree.

Example reverse dates (computed)

EDD Sep 1, 2026, 28-day

Conception Dec 9, 2025

LMP Nov 25, 2025

Same EDD, 35-day cycle

LMP Nov 18, 2025

Conception unchanged · −287 d to LMP

EDD Sep 1, 24-day cycle

LMP Nov 29, 2025

−276 days back

Birth date May 10, 2026

Conception Aug 17, 2025

LMP Aug 3, 2025

266 vs 280 days — formulas

Same rules as pregnancy wheels, run backward from EDD or birth date.

Step	Formula	Example
Estimated conception	EDD − 266 days	EDD Sep 1, 2026 → conception Dec 9, 2025
LMP (cycle-adjusted)	EDD − (280 + (C − 28)) days	C=35 → subtract 287 days from EDD
28-day shortcut	LMP = EDD − 280 when C = 28	Same as Naegele forward rule reversed
Fertile window (teaching)	Conception − 4 days through conception + 1 day	Sperm survival ~3–5 days pre-ovulation

Cycle length vs LMP (EDD Sep 1, 2026)

Cycle	Days back to LMP	Est. LMP	Est. conception
24 days	276	Nov 29, 2025	Dec 9, 2025
28 days	280	Nov 25, 2025	Dec 9, 2025
32 days	284	Nov 21, 2025	Dec 9, 2025
35 days	287	Nov 18, 2025	Dec 9, 2025

Conception identical across cycles in this model; only LMP shifts.

Gestational landmarks from LMP

Milestone	Weeks	Days after LMP
End of 1st trimester	13 weeks	91
End of 2nd trimester	27 weeks	189
Estimated due date (40 wk from LMP)	40 weeks	280

Dating method comparison

Method	Accuracy	Note
LMP + cycle length	Moderate	Assumes regular cycles and known LMP
First-trimester ultrasound	High	Preferred if LMP uncertain or >5–7 day discrepancy
Reverse from EDD (this tool)	Same as forward LMP rule	Estimate only—confirm with records
IVF transfer date	Very high	Use IVF due-date calculator—not 266-day guess

280 vs 266 — two-week rule

Concept	Count from	Typical span
Clinical gestational age	From LMP (day 1 of bleeding)	40 weeks to EDD
Fetal / conception age	From fertilization	~38 weeks to delivery
The "2-week gap"	LMP is ~14 days before conception on 28-day cycle	Explains 280 vs 266

Worked example: EDD Sep 1, 2026, 28-day cycle

Anchor (EDD or birth date) = 2026-09-01
Conception ≈ EDD − 266 = 2025-12-09 (Tuesday, December 9, 2025)
LMP ≈ EDD − (280 + (28 − 28)) = EDD − 280 = 2025-11-25
Fertile window (teaching) = Dec 5, 2025 – Dec 10, 2025
End 1st trimester ≈ LMP + 91 = 2026-02-24
End 2nd trimester ≈ LMP + 189 = 2026-06-02

Switch cycle length to 32 or 35 in the calculator—only the LMP line moves earlier; conception stays Dec 9, 2025.

EDD or birth date

Same backward math

Use provider EDD when available; birth date works when delivery already occurred—remember only ~5% deliver on the original EDD.

Cycle adjustment

LMP only

Longer cycles push estimated LMP further back from EDD; conception anchor stays 266 days in this teaching model.

Not for IVF

Use transfer-based dating

IVF pregnancies have known fertilization/transfer timing—use the IVF due date calculator instead.

Educational disclaimer

This reverse due date calculator applies standard 266/280-day obstetric rules for learning and timeline curiosity. It does not establish legal paternity, replace ultrasound biometry, or provide prenatal medical advice.

ACOG — pregnancy dating & prenatal resources
If ultrasound and LMP-based reverse dates disagree by more than ~1 week, trust clinician-documented ultrasound dating.

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Obstetrics & maternity

Reverse Due Date Calculator — Conception & LMP From EDD

Last updated: June 5, 2026

Conception date + fertile window (5-day teaching band)

Cycle-adjusted LMP and trimester milestone dates

Comparison tables and step-by-step formula examples

Fertile window vs one conception day

Sperm can survive several days before ovulation. Intercourse in the days leading up to the estimated conception date can still explain the same pregnancy.

Reverse Due Date Calculator

Work backward from EDD or birth date to estimated conception, LMP, and trimester milestones.

Due date or birth date

Average cycle length (days)

Estimated conception

Tuesday, December 9, 2025

Fertile window: Dec 5, 2025 – Dec 10, 2025

Timeline

LMP (est.)Nov 25, 2025

End 1st trimesterFeb 24, 2026

End 2nd trimesterJun 2, 2026

EDD / anchorSep 1, 2026

Clinical estimates using 266- and 280-day rules. Ultrasound dating overrides when records disagree.

Example reverse dates (computed)

EDD Sep 1, 2026, 28-day

Conception Dec 9, 2025

LMP Nov 25, 2025

Same EDD, 35-day cycle

LMP Nov 18, 2025

Conception unchanged · −287 d to LMP

EDD Sep 1, 24-day cycle

LMP Nov 29, 2025

−276 days back

Birth date May 10, 2026

Conception Aug 17, 2025

LMP Aug 3, 2025

266 vs 280 days — formulas

Same rules as pregnancy wheels, run backward from EDD or birth date.

Step	Formula	Example
Estimated conception	EDD − 266 days	EDD Sep 1, 2026 → conception Dec 9, 2025
LMP (cycle-adjusted)	EDD − (280 + (C − 28)) days	C=35 → subtract 287 days from EDD
28-day shortcut	LMP = EDD − 280 when C = 28	Same as Naegele forward rule reversed
Fertile window (teaching)	Conception − 4 days through conception + 1 day	Sperm survival ~3–5 days pre-ovulation

Cycle length vs LMP (EDD Sep 1, 2026)

Cycle	Days back to LMP	Est. LMP	Est. conception
24 days	276	Nov 29, 2025	Dec 9, 2025
28 days	280	Nov 25, 2025	Dec 9, 2025
32 days	284	Nov 21, 2025	Dec 9, 2025
35 days	287	Nov 18, 2025	Dec 9, 2025

Conception identical across cycles in this model; only LMP shifts.

Gestational landmarks from LMP

Milestone	Weeks	Days after LMP
End of 1st trimester	13 weeks	91
End of 2nd trimester	27 weeks	189
Estimated due date (40 wk from LMP)	40 weeks	280

Dating method comparison

Method	Accuracy	Note
LMP + cycle length	Moderate	Assumes regular cycles and known LMP
First-trimester ultrasound	High	Preferred if LMP uncertain or >5–7 day discrepancy
Reverse from EDD (this tool)	Same as forward LMP rule	Estimate only—confirm with records
IVF transfer date	Very high	Use IVF due-date calculator—not 266-day guess

280 vs 266 — two-week rule

Concept	Count from	Typical span
Clinical gestational age	From LMP (day 1 of bleeding)	40 weeks to EDD
Fetal / conception age	From fertilization	~38 weeks to delivery
The "2-week gap"	LMP is ~14 days before conception on 28-day cycle	Explains 280 vs 266

Worked example: EDD Sep 1, 2026, 28-day cycle

Anchor (EDD or birth date) = 2026-09-01
Conception ≈ EDD − 266 = 2025-12-09 (Tuesday, December 9, 2025)
LMP ≈ EDD − (280 + (28 − 28)) = EDD − 280 = 2025-11-25
Fertile window (teaching) = Dec 5, 2025 – Dec 10, 2025
End 1st trimester ≈ LMP + 91 = 2026-02-24
End 2nd trimester ≈ LMP + 189 = 2026-06-02

Switch cycle length to 32 or 35 in the calculator—only the LMP line moves earlier; conception stays Dec 9, 2025.

EDD or birth date

Same backward math

Use provider EDD when available; birth date works when delivery already occurred—remember only ~5% deliver on the original EDD.

Cycle adjustment

LMP only

Longer cycles push estimated LMP further back from EDD; conception anchor stays 266 days in this teaching model.

Not for IVF

Use transfer-based dating

IVF pregnancies have known fertilization/transfer timing—use the IVF due date calculator instead.

Educational disclaimer

ACOG — pregnancy dating & prenatal resources
If ultrasound and LMP-based reverse dates disagree by more than ~1 week, trust clinician-documented ultrasound dating.