Estimate when you likely conceived, working backward from either your due date or your last period.
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How conception date is estimated
If you know your due date, conception is estimated as 266 days before it — since a full-term pregnancy is typically counted as 280 days from the last menstrual period, and conception usually happens about two weeks after that. If you know your last period date instead, conception is estimated using your cycle length: ovulation (and therefore conception) usually occurs around 14 days before the next period, so a longer or shorter cycle shifts the estimate accordingly.
Why this is a window, not a single day
Sperm can survive for several days, and ovulation timing can vary even in regular cycles, so conception is best understood as a several-day window rather than one exact date. This calculator shows both a single best estimate and a realistic window around it.
Frequently asked questions
Which method is more accurate — due date or LMP?
If your due date came from an early ultrasound, that method is usually more accurate than counting from your last period, since ultrasound dating adjusts for irregular cycles.
Can this tell me the exact day I conceived?
No — it gives an estimated window, since ovulation timing varies and sperm can fertilize an egg several days after intercourse.
How is this different from an ovulation calculator?
An ovulation calculator looks forward from your last period to predict a future fertile window. This tool works backward from a known due date or period to estimate when conception already happened.
Medical note: This calculator gives a general estimate only and is not medical advice. A conception date is an estimate, and early ultrasound dating from your doctor is more reliable. Contact your doctor or midwife about dating, scans and any concerns.