Work out when you probably conceived from your due date.
Conception happens about 266 days (38 weeks) before the due date. For a due date of 22 October 2026 that is around Thursday, 29 January 2026, so intercourse most likely happened in the week before.
If you know your due date, you can estimate when conception happened. This calculator subtracts 266 days from your due date to find the likely day of ovulation and conception, shows the 5-day-before to 1-day-after window in which intercourse could have led to the pregnancy, and your estimated last period date.
Because sperm can survive up to five days and babies are rarely born exactly on their due dates, the true date may be a few days either side — the page also shows the range if birth occurs at 37 or 42 weeks. See the Conception Calculator to predict conception from your cycle and the Due Date Calculator.
The calculator applies the standard published formulas step by step.
| Item | Value |
|---|---|
| Due date | Thursday, 22 October 2026 |
| Estimated first day of last period | Thursday, 15 January 2026 |
| Estimated ovulation / conception | Thursday, 29 January 2026 |
| Most likely window for intercourse (5 days before to 1 day after ovulation) | Saturday, 24 January 2026 – Friday, 30 January 2026 |
| If the baby arrives at 37 weeks, conception was around | Thursday, 8 January 2026 |
| If the baby arrives at 42 weeks, conception was around | Thursday, 12 February 2026 |
| Due date | Conception around |
|---|---|
| Due 1 Sep 2026 | Tuesday, 9 December 2025 |
| Due 22 Oct 2026 | Thursday, 29 January 2026 |
| Due 25 Dec 2026 | Friday, 3 April 2026 |
| Due 1 Mar 2027 | Monday, 8 June 2026 |
Subtract 266 days (38 weeks) from your due date. That gives the most likely date of ovulation and fertilisation.
It is an estimate. Ovulation timing varies, and doctors' due dates are usually based on an assumed 28-day cycle.
Sperm can survive for up to 5 days in the body, while the egg lives about 24 hours, so intercourse in the 5 days before ovulation can cause pregnancy.
Birth at 37 weeks suggests conception three weeks earlier than for a 40-week birth in the calculator's range; at 42 weeks, two weeks later.
An early scan dates the pregnancy to within a few days, but it still cannot tell exactly when intercourse took place.
The pregnancy is counted from the last period, about two weeks before conception.
It uses the published formula: conception ≈ due date − 266 days LMP ≈ due date − 280 days The full method and a worked example are shown on this page.
No. The calculator gives general estimates for information and education. It cannot diagnose or treat anything, so please speak to a doctor or qualified professional about your own health.
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