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Estimate when conception likely occurred and when your last period probably started—by counting backward from an estimated due date (EDD) or a baby's birth date. Adjusts for your average cycle length using the same 266- and 280-day rules clinicians use for rough dating (ultrasound and provider input stay the gold standard).
Last updated: March 21, 2026
Sperm can survive several days before ovulation. Intercourse in the days leading up to the estimated conception date can still explain the same pregnancy—treat the output as a timeline guide, not a single definitive timestamp.
Disclaimer: This tool uses the standard 280-day physiological gestation rule. Because pregnancy lengths naturally vary and ovulation exact timing shifts, these dates are clinical estimates, not guarantees.
Built for curiosity and planning: clear outputs that mirror common obstetric counting, plus cycle-aware LMP so longer or shorter cycles are not forced into a one-size-fits-all day-14 assumption for period start.
A reverse due date (or backward) calculation starts from a known or guessed end point—usually an estimated due date (EDD) or a birth date—and estimates what came before: most importantly approximate conception and the first day of the last menstrual period (LMP) used in clinical pregnancy counting.
It matters for personal timeline questions ("when might we have conceived?"), for filling in history forms, and for understanding why providers say "40 weeks" when fetal age from conception is closer to 38 weeks. It does not replace ultrasound dating, IVF transfer math, or individualized medical advice.
Biological variation is real: ovulation shifts, late or early periods, and early or late births all widen the error bars. Treat every output as an estimate centered on population-average rules, not a legal or medical determination.
This calculator uses the same backbone as many pregnancy wheels: fixed day counts from the EDD, plus a simple correction when your average cycle is not 28 days.
In this tool, 266 days before the EDD is the same for every cycle length, matching the form logic: cycle length moves LMP, not this conception anchor.
End of first trimester ≈ LMP + 91 days (through week 13). End of second trimester ≈ LMP + 189 days (through week 27). These are calendar checkpoints many patients recognize on apps and visit schedules.
Illustrative dates using the same rules as the calculator. Replace with your own EDD to replicate.
Tables use sample EDD Sep 1, 2026. Conception date is identical across cycles here; only LMP moves when cycle length changes.
| Avg. cycle (days) | Days EDD → LMP | Est. LMP | Est. conception |
|---|---|---|---|
| 24 | 276 | Nov 29, 2025 | Dec 9, 2025 |
| 28 | 280 | Nov 25, 2025 | Dec 9, 2025 |
| 32 | 284 | Nov 21, 2025 | Dec 9, 2025 |
| 35 | 287 | Nov 18, 2025 | Dec 9, 2025 |
| Milestone | Weeks (approx.) | Days after LMP |
|---|---|---|
| End of 1st trimester (after week 13) | 13 wk | 91 |
| End of 2nd trimester (after week 27) | 27 wk | 189 |
| Estimated due date (from LMP, 28-day rule) | 40 wk | 280 |
Set due date to 2026-09-01 and cycle length to 28. You should see LMP around Nov 25, 2025 and estimated conception around Dec 9, 2025—then switch cycle length to 32 and watch only the LMP date move earlier on the timeline.
Help friends map backward from a due date to conception and LMP estimates.
Suggested hashtags: #Pregnancy #Maternity #Conception #MomLife