Reverse due date calculator
Work backward from your due date to a conception window and LMP estimate — honest ranges, not an exact hour. Estimate only — not medical advice.
We’ll work backward to the days conception most likely fell within, plus the last-period date clinics count from.
Enter a due date to see the conception window.
What this page does
Have a due date and want the days behind it? Reverse dating starts from the EDD and walks backward: conception is usually treated as about 266 days before the due date, shown as a window rather than a timestamp, and LMP is often estimated as about 280 days before.
This page is built for planning and curiosity. It deliberately resists "exact conception Tuesday at 3:12pm" theater. A due date is already an estimate — sometimes rounded, sometimes scan-adjusted — so working backward inherits that uncertainty. A several-day conception window is the honest product.
People land here when two apps disagree, when a portal only ever printed an EDD, or when someone asks "so when did we conceive?" and the truthful answer is a range. If you have an IVF transfer date, forward dating from transfer is usually stronger than reversing a rounded EDD — use both as a cross-check, not as a courtroom exhibit.
The math runs in your browser. It will not settle disputes, rewrite an ultrasound report, or invent a period diary. It gives you a window and an LMP-style anchor you can explain without fake precision.
When to use
Two apps disagree about when things started. You only ever received an EDD and need a conception-range explanation for a form or a conversation. You are reconciling IVF paperwork with a tool that only speaks LMP.
Also useful when someone asks "so when did we conceive?" and the honest answer is a several-day window. Leave planners and registry wizards sometimes ask for conception timing; a range keeps you from inventing false precision you will later have to unteach. Partners who only saw an EDD on MyChart often need the backward walk spelled out once.
When not to
Do not use reverse math as legal proof of a conception date. Do not use it to accuse anyone. Prefer IVF forward dating when you have a transfer date — that is stronger than reversing a rounded EDD.
If a dating ultrasound changed your EDD, reverse from the current official EDD, not an old sticky note. Emergency care decisions do not belong on a reverse calendar widget. Do not treat the LMP estimate as proof of menses that day.
Assumptions
Conception midpoint ≈ EDD − 266 days, shown with a ±3 day window by default. LMP estimate ≈ EDD − 280 days.
These are standard averages. They do not absorb every clinic's scan adjustment history, irregular-cycle edge cases, or IVF culture-day nuance. The window width is a honesty choice — not a claim that biology always fits ±3 perfectly. Sperm survival across multiple days is another reason a single circled day is overfit.
Examples
EDD November 26, 2026 → Conception window ~March 7–13, 2026 · LMP ~Feb 19, 2026. Midpoint March 10 (EDD − 266) with ±3 days; LMP = EDD − 280.
EDD June 1, 2027 → Conception midpoint ~Sept 8, 2026. Window spans a few days around the midpoint.
EDD January 15, 2027 → LMP estimate ~April 10, 2026. Useful for forms; not proof of menses that day.
Gotchas
Exact-hour conception claims from a due date alone are not honest.
IVF transfer dating beats reverse guessing when transfer day is known.
If the EDD came from an early ultrasound, reverse numbers are approximate context, not a correction tool.
A week-wide conception window is a feature, not a bug.
LMP estimate here is calendar math, not a remembered period diary.
Reversing an outdated sticky-note EDD after a scan update will resurrect a wrong window.
Sperm can survive multiple days; that is one reason a single circled day is overfit.
How this is calculated
Range-first on purpose: a due date is already an estimate, so working backward inherits that uncertainty.
Enter the EDD you actually trust — usually the portal or clinic number — then read the conception window as the primary output. The LMP estimate is secondary paperwork context, not a memoir of menses. If you later need to paste an LMP into a stubborn form, the dedicated LMP-equivalent page uses the same 280-day idea with paperwork framing.
- Enter the estimated due date you actually trust (portal/clinic).
- Read the conception window as the primary output.
- Note the LMP estimate as secondary paperwork context.
- If you have a transfer date, verify forward with the IVF calculator instead of relying only on reverse math.
- If the EDD later changes after a scan, rerun reverse dating from the new EDD.
- Treat the result as a planning window, not a courtroom exhibit.
conception midpoint ≈ EDD − 266 (± window) LMP estimate ≈ EDD − 280
Related calculators
Conception calculator and "when did I conceive" pages cover related intents with different framing — broader inputs versus the blunt question wording. IVF LMP-equivalent is for forms that demand a period field and want that job labeled clearly.
If you still have transfer day, run the IVF due date tools forward and treat reverse dating as a cross-check, not the primary source of truth.
See also: conception calculator, when did i conceive calculator, ivf lmp equivalent calculator, and ivf due date calculator.
FAQ
- Can you tell me the exact day I conceived?
- No honest calculator can pin that from an EDD alone. The dates support a window of a few days. Anyone offering an exact minute from a due date is overselling calendar math.
- Why show a last-period date too?
- Clinics and forms often count pregnancy from an LMP-style anchor, so the estimate helps line up paperwork. It is not proof you menstruated that day.
- How is this different from IVF dating?
- IVF forward dating starts from transfer and embryo age. Reverse dating starts from an EDD you already have. When both exist, forward transfer math is usually the clearer story.
- Why ±3 days?
- To keep the answer honest. False precision is worse than a clear range. Due dates and fertilization timing both carry uncertainty; the window makes that visible.
- Do my dates leave my device?
- No. Calculation stays in your browser on this site.
- What if my EDD changed after an ultrasound?
- Reverse from the current official EDD. Old sticky notes reverse into old stories.
- Is the LMP estimate the same as an IVF LMP-equivalent?
- Same 280-day idea when starting from EDD. The LMP-equivalent page focuses on feeding forms; this page leads with conception window and treats LMP as secondary.
- Is this medical or legal advice?
- Neither. Planning estimate only — not proof for disputes.