Conception Date Calculator

This calculator runs the pregnancy clock in reverse. Give it a due date and it subtracts 266 days to land on the most likely conception date. Give it the first day of your last period (LMP) and it adds 14 days. Both routes assume a textbook 28-day cycle with ovulation on day 14, which is why the result is a single date and not a range. Knowing this number is useful for matching ultrasound dating, working out which night actually counts and answering a partner's quietly persistent maths.

Worked example. Your due date is 1 January 2027. Subtract 266 days and conception was around 11 April 2026. Or: your last period started on 28 March 2026, add 14 days, conception around 11 April 2026. The two methods agree because the standard pregnancy duration of 280 days from LMP is exactly 266 days from conception plus 14 days from LMP to ovulation. Use the [pregnancy due date calculator](/pregnancy-due-date-calculator) if you need to go the other way and pin down the due date first.

Sperm survive in the female reproductive tract for up to 5 days. The egg lives 12 to 24 hours after ovulation. So the conception date is not necessarily the same as the date of intercourse: a Saturday encounter can result in fertilisation on the following Wednesday if ovulation arrives later. The calculator gives you the most likely fertilisation day, not the most likely fun day. For couples trying to confirm paternity timing or date a complicated pregnancy, the realistic intercourse window is conception date minus 5 days through to conception date plus 1 day.

Cycle length matters too. The 14-day estimate assumes ovulation is exactly halfway through the cycle, but for someone on a 35-day cycle ovulation is closer to day 21, not day 14. If your cycles are reliably longer or shorter than 28 days, the LMP-based estimate from this tool will be off by the difference. The conception date calculation from due date is more reliable in that case, because the due date itself was usually adjusted for cycle length at your dating scan.

An early ultrasound (8 to 13 weeks) measures crown-rump length and works out how far along the embryo is. The result is a 'gestational age' counted from LMP, so subtract 14 days to get a conception estimate. If the scan says you are 9 weeks 4 days on the day of the scan, you conceived approximately 9 weeks 4 days minus 14 days = 7 weeks 4 days before scan day. Most due dates are revised at the dating scan if the scan-based age differs from LMP-based age by more than 5 to 7 days, because the scan is more reliable than memory of the last period.

If the calculator says one date and the scan says another, trust the scan. Your last period might have been a week earlier or later than you remember; ovulation might have been delayed by stress or illness. Scan dating is accurate to within 3 to 5 days in the first trimester. After 14 weeks scan dating gets less reliable because growth rates vary. The [implantation calculator](/implantation-calculator) projects the next milestone forward from this conception date, and the [fertile window calculator](/fertile-window-calculator) shows the broader window of intercourse dates that could explain the same conception.

Three real reasons people work this out. First, pinning down whose pregnancy it is when there are two possible partners and the last period predates both. Second, matching the conception window to a specific event - a wedding, a holiday, a fertility treatment cycle - so the rough timing makes sense. Third, working out whether a positive test is consistent with what you remember; a 4-week pregnancy means conception roughly 2 weeks ago, so a date 5 weeks ago should not have produced a positive test until last week.

This is a planning tool, not a diagnostic one. It cannot prove paternity (only DNA testing can), it cannot account for an irregular cycle without more information than 'last period date', and it does not replace medical advice. If the answer here matters legally, talk to your GP and ask for a dating scan as early as possible; the NHS offers one between 8 and 14 weeks and that is the date a court will rely on, not a calculator.