How conception dating works
Conception happens when a sperm fertilises an egg, and that can only occur in a short span around ovulation — the moment an ovary releases an egg. The egg itself survives for only about 24 hours after release, so fertilisation must happen on ovulation day or very shortly after. This calculator therefore dates conception from estimated ovulation. In a textbook 28-day cycle ovulation falls on day 14, but the part of the cycle before ovulation is the part that stretches and shrinks, while the phase after ovulation stays fairly steady at about 14 days. That is why the formula works from your cycle length: ovulation is estimated at cycle length minus 14 days after the first day of your last period. A 28-day cycle gives day 14; a 32-day cycle gives day 18; a 24-day cycle gives day 10.
The formulas this calculator uses
The 266-day rule is the mirror image of the familiar 280-day pregnancy count. A pregnancy is counted as 280 days from the last period, but conception happens roughly 14 days into that count, so a due date sits about 266 days after conception. If you already know your due date — from a scan, for example — subtracting 266 days lands on the estimated conception date, and subtracting the full 280 days recovers the matching last period date. The likely window shown is the estimated day plus and minus two days, a deliberate admission that calendar dating is approximate.
Conception date vs fertile window — they are not the same thing
Two different questions are often mixed up. "When did I conceive?" asks about the single day fertilisation happened, which is ovulation day, give or take a day. "When could the sex that led to this pregnancy have happened?" covers a much wider span, because sperm can survive inside the reproductive tract for up to about five days, waiting for the egg to arrive. Sex five days before ovulation can therefore lead to conception just as surely as sex on the day itself, even though conception itself still happened on ovulation day. So if you are working backwards to a particular day, the honest answer is the fertile window — roughly five days before conception through a day after — while the conception date itself stays pinned to ovulation. The breakdown table on this page shows both, including the earliest day intercourse could realistically have led to this conception.
Why an ultrasound date can differ
Every date here rests on assumptions: that your cycle is the length you entered, that ovulation followed the 14-day luteal pattern, and that the period date is remembered exactly. Real cycles vary, ovulation drifts by a few days even in people who feel clockwork-regular, and period dates are easy to misremember. An early ultrasound measures the developing baby directly, and in the first trimester that measurement is a more accurate clock than the calendar, which is why clinics routinely revise dates after a scan — sometimes by nearly a week. A difference between this page and a scan date does not mean either is "wrong" in a worrying way; it means a direct measurement has replaced an estimate. When the two disagree, plan around the date your care team gives you. All results here are estimates only and are not medical advice.
Worked example
Last period starts September 15, 2026, with a 28-day cycle. Conception = Sep 15 + (28 − 14) = Sep 15 + 14 days = September 29, 2026. The likely window is two days either side: September 27 to October 1, 2026. Due date = Sep 15 + 280 days = June 22, 2027. Working backwards from that due date instead: June 22, 2027 − 266 days = September 29, 2026 — the same conception date, as it should be.
How to use this calculator
- Choose whether you are calculating from your last period or from a due date you already know.
- For the period route, enter the first day of your last period and your average cycle length.
- For the due date route, enter the due date — the cycle field is not used, and a standard 28-day cycle is assumed.
- Read the estimated conception date in the headline panel.
- Use the window, not just the single day, when the exact timing matters, and compare with any ultrasound dating you have been given.
How sure can a conception date ever be?
Only people who conceived through timed medical procedures, such as IVF with a known transfer date, can usually name the day with confidence. For everyone else, a conception date is a well-reasoned estimate built on averages: the average luteal phase, the average cycle, dates as remembered. Even ovulation predictor kits and temperature charts narrow the day down rather than prove it, because they detect the hormonal signal around ovulation, not the moment of fertilisation itself. That uncertainty is normal and, for most purposes — following a pregnancy week by week, planning appointments, satisfying curiosity about timing — a window of a few days makes no practical difference at all.
Where timing genuinely matters, treat this page as a starting point for a conversation with a qualified health professional, who can combine scan measurements with your history. And hold the wider context: a due date is the centre of a several-week window in which birth is completely normal, not a deadline. This calculator gives estimates only and is not medical advice; it cannot confirm a pregnancy, rule one out, or date one as reliably as an ultrasound. If you have pain, bleeding or any urgent concern, seek care promptly rather than consulting any calculator.
Important note
For information only — not medical advice. This calculator uses standard published formulas to estimate values for general education and planning. It does not diagnose, treat, or replace the advice of a doctor or other qualified health professional. If you have a health concern, are pregnant, or are making decisions about treatment, medication, or a major diet or exercise change, please talk to a healthcare professional first.