Pregnancy Conception Calculator

Count backwards from a due date to estimate when conception happened: the due date minus 266 days, plus the likely window of a few days either side.

Pregnancy Conception Calculator

How it works: A typical pregnancy lasts about 266 days from conception to the due date, so counting back 266 days gives an estimated conception date. The likely window runs from 5 days before to 2 days after that date, because cycles and timing vary.

How the Pregnancy Conception Calculator Works

This calculator estimates when conception most likely happened, working backwards from the due date only. Conception usually happens around ovulation, roughly 14 days after the last menstrual period in a regular 28-day cycle, so the calculator’s core step is simple: conception date ≈ due date − 266 days (38 weeks), because a full-term pregnancy is 266 days from conception but 280 days from the last menstrual period. Only one input is needed — the due date itself, entered exactly as it appears on your scan report or clinic paperwork. There is no option to enter a last-period (LMP) date on this page; if all you know is your LMP, run the Pregnancy Calculator first to turn it into a due date.

Because sperm can survive up to five days and the egg about 24 hours, conception is not a single day but a window: the fertile window runs from five days before ovulation through ovulation day itself. The calculator therefore shows both the most likely conception day (ovulation day) and the full fertile window around it, so you can see the range of possible dates rather than a single point. From the due date, it works the other way: subtract 266 days to reach the likely conception day, then place the fertile window around it.

All results assume a regular 28-day cycle with ovulation on day 14; longer or shorter cycles move ovulation and shift every date the calculator produces.

How to Use This Calculator

  1. Enter your due date, exactly as it appears on your scan report or clinic paperwork.
  2. Enter that date.
  3. Press Calculate to see the most likely conception date and the fertile window.
  4. Use the window, not just the single day, when narrowing down possible dates — conception can happen on any day in it.
  5. If your cycles are irregular, treat the result as approximate and confirm with a clinician or an early scan.

Example

If the due date is 8 December 2026, the most likely conception date is 15 March 2026 (8 December − 266 days), and the fertile window is roughly 10–14 March 2026 — the five days before ovulation plus ovulation day.

There is no option to enter a last-period (LMP) date on this page; if all you know is your LMP, run the Pregnancy Calculator first to turn it into a due date.

Understanding Your Result

The result is a window, not a verdict. Even with a perfectly regular cycle, ovulation can shift by a day or two, and sperm survival means conception can precede ovulation by up to five days — so the calculator’s single “most likely” day sits inside a range, and any day in that range is a plausible conception date. This matters most when the date is used for personal or legal reasons: the calculator narrows the possibilities, but it cannot prove a single day. An early ultrasound (dating scan at 8–12 weeks) measures the embryo directly and is the most accurate way to pin down gestational age.

The 14-day assumption is the other limit. In a 35-day cycle, ovulation typically falls around day 21, not day 14, so the LMP-based estimate would be a week early; in a 21-day cycle it would be a week late. If you track ovulation — with temperature charts, ovulation predictor kits or cervical mucus — use your actual ovulation day instead of the calculator’s estimate: add it to the fertile window directly, and count the due date as ovulation + 266 days. For irregular cycles, the scan, not the calendar, is the reliable answer.

Pregnancy Conception Calculator FAQs

How accurate is the conception date estimate?

It is an estimate based on a regular 28-day cycle with ovulation on day 14. The fertile window (five days before ovulation plus ovulation day) is a more honest answer than a single day, and an early ultrasound is more accurate still.

Can conception happen outside the fertile window?

Very rarely. Sperm survive up to five days and the egg about a day, so conception outside the six-day window is unlikely — but ovulation itself can shift, which moves the whole window.

Why is 266 days used, not 280 days?

Because 280 days counts from the last menstrual period, while 266 days counts from conception, and the 14-day gap between the last period and ovulation accounts for the difference.

Does this calculator work for IVF?

For IVF, the conception (fertilisation) date is known exactly — it is the egg retrieval/fertilisation date — so you do not need to estimate it. The due date is then fertilisation + 266 days, or transfer date adjusted for embryo age.

What if my cycles are irregular?

The 14-day assumption breaks down: ovulation may fall much earlier or later than day 14. Use a tracked ovulation day if you have one, or confirm dates with an early ultrasound.

Related calculators: Pregnancy Calculator, Due Date Calculator, Date Calculator.