When did I conceive? What the dates can and can't tell you
People come to this question from very different places — curiosity about which week it happened, medical forms asking for a conception date, sometimes questions with real stakes attached. So it's worth being precise about what's knowable: unless you tracked ovulation closely or conceived through IVF with a known transfer date, conception can only ever be estimated as a window of several days, not pinned to one.
The estimate itself is solid math. Conception happens at ovulation, and ovulation sits roughly 14 days before the next period would have arrived. This conception calculator works from either end: give it your last period and it projects forward; give it a due date and it counts 266 days back.
The two ways the calculator estimates it
From a due date: standard pregnancy dating runs 280 days from last period to due date, and conception follows about two weeks after the period starts — so conception lands roughly 266 days (38 weeks) before the due date. A due date of, say, October 15 points to a conception window around January 22.
From your last period: the calculator adds your cycle length minus 14 days. That subtlety matters more than it looks — with a 28-day cycle, ovulation is around day 14, but with a 35-day cycle it's around day 21. The 'minus 14' rule holds because the second half of the cycle (the luteal phase) is the stable part; it's the first half that stretches with longer cycles.
Around the estimated day, biology adds its own margin: sperm survive up to five days, and the egg is viable for about one. That six-day fertile window is the honest answer to 'when' — intercourse days before ovulation can still be the origin.
How much to trust the estimate
For most purposes — pregnancy milestones, idle curiosity — a few days either way is fine, and this estimate delivers that for regular cycles. Where cycles are irregular, ovulation moves unpredictably and the window widens. And if the question behind your question is paternity in a case where dates are close together, calendar math genuinely cannot settle it; DNA testing is the only reliable answer, and it's worth saying that plainly rather than letting a calculator imply more precision than exists.
For the forward-looking version of this math — predicting fertile days for conceiving — the ovulation calculator is the right tool, and the due date calculator handles the finish line.
Frequently asked questions about conception dates
When does conception usually occur?
Conception typically happens when sperm fertilises an egg during the fertile window — often about 14 days after the start of the last period in a 28-day cycle, but timing varies.
Can I know the exact conception date?
Usually only an estimate unless you tracked ovulation precisely or had assisted reproduction with a known transfer date.
Is this medical advice?
No. For pregnancy, fertility, or health concerns, speak to your doctor, midwife, or OB-GYN.
Sources and further reading
- ACOG — Methods for estimating the due date
- Wilcox AJ et al. — Timing of intercourse and probability of conception (NEJM, 1995)
- MedlinePlus — Pregnancy
This page is for general information only and is not medical advice. Always confirm pregnancy dating and care decisions with your doctor or midwife.