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.
Names and Family Data Desk, BabyData
BabyData's editorial desk builds and documents the tools, citing the underlying rule and the official UK dataset behind every number. Pregnancy-related tools are editorially reviewed against NHS and gov.uk guidance before publication.
Last reviewed: 12 June 2026