Conception date calculator
Your likely conception date is about 14 days after the first day of your last period, which is the due date minus 266 days. BabyData works it back from either your due date or your last period and shows a likely window of a few days, because the exact day depends on when you ovulated. This is an estimate based on how the NHS dates pregnancy, not medical advice and not a paternity or legal date. Pregnancy content here is informational only and reviewed by our editorial team against NHS and gov.uk guidance.
Estimate the conception date
Worked back from your due date or last period, with a likely window.
How this is worked out
Pregnancy is dated from the first day of your last period, and ovulation (when conception can happen) is about 14 days later. So conception is roughly:
Conception ≈ LMP + 14 days
Conception ≈ due date − 266 days
From a due date, the count is the due date minus 266 days (38 weeks), because the standard 280-day due date is measured from the LMP, which is about 14 days before conception. We show a likely window of a few days either side, because the exact day depends on your cycle.
An estimate, not medical advice and not a paternity or legal date. The actual day of conception can vary because cycle length and ovulation timing differ. Editorially reviewed against NHS and gov.uk guidance.
Common questions
How do you work out the conception date?
Your likely conception date is estimated as your due date minus 266 days, or about 14 days after the first day of your last period, because pregnancy is dated from the last period and ovulation follows roughly two weeks later. The tool shows a window of a few days because the exact day depends on when you ovulated.
Why is conception counted as 14 days after my last period?
Because ovulation, when conception can happen, usually occurs about 14 days after the first day of your last period on an average 28-day cycle. That is why a 280-day due date measured from the last period works out to about 266 days from conception.
How accurate is this if my cycle is irregular?
It can be out by a week or more. The method assumes a regular 28-day cycle with ovulation around day 14, so when ovulation is later or unpredictable the estimate is less reliable. A dating scan, which measures the baby directly, is preferred when cycles are irregular. Treat any date before your scan as provisional.
Is the conception date exact or legal?
No. It is an estimate based on how the NHS dates pregnancy, not a paternity or legal date, and the true day can vary with your cycle. This is general information, not medical advice, so speak to your midwife or GP to confirm your dates.
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Editor, BabyData
Oliver Mackman is the named editor of BabyData and a director of Best Business Loans Ltd (Companies House 16833937), the company that operates the site. He is accountable for its editorial standards, sourcing and corrections, and requires every number to cite the underlying rule and the official UK dataset behind it, with pregnancy tools reviewed against NHS and gov.uk guidance before publication.
Last reviewed: 12 June 2026