Conception Date Calculator
Estimate your conception date and fertile window from your last period (adjusted for cycle length) or from your due date, with an implantation window.
How conception is estimated
Conception happens at ovulation, and ovulation usually occurs about 14 days before the next period
— regardless of how long the cycle is. From your last period, with cycle length C:
Conception ≈ LMP + (C − 14) days
From a due date, the arithmetic runs backwards through the standard 40-week pregnancy model: conception ≈ due
date − 266 days, and the last period began around due date − 280 days.
Why it can only ever be an estimate
Sperm can survive in the reproductive tract for up to five days, so intercourse days before
ovulation can still lead to conception on ovulation day — and the egg itself remains viable for about 24 hours.
That gives a fertile window of roughly six days, which this calculator shows as ovulation ±3 days. On top of
that, ovulation timing itself shifts from cycle to cycle, even in regular cycles. Treat the result as the centre
of a range, not a pinpoint.
Worked example
Last period started 1 June 2026 with a 30-day cycle: ovulation is expected around day 16, so
conception ≈ 17 June 2026. The fertile window spans about 14–20 June, implantation would follow
around 23–29 June, and the resulting due date is 10 March 2027 (conception + 266 days).
Implantation and pregnancy tests
The fertilised egg implants in the uterine lining about 6–12 days after conception, which is when hCG — the
hormone pregnancy tests detect — starts being produced. That is why testing before the missed period often gives
a false negative even when conception has occurred.
Frequently asked questions
Can this tell me the exact day I conceived?
No tool can. Sperm survive up to five days in the reproductive tract and the egg is viable for about a day after ovulation, so conception can result from intercourse on any of roughly six days. The date shown is the most likely single day — ovulation — with the fertile window around it.
How is the conception date estimated from my last period?
Ovulation typically happens about 14 days before the next period. With a cycle of C days, that is day C − 14: for a 28-day cycle, day 14; for a 32-day cycle, day 18. Conception is assumed to happen on or very close to that day.
How is conception estimated from a due date?
Working backwards from the standard 40-week model: conception ≈ due date − 266 days (38 weeks), and the last period began around due date − 280 days. This works for a past due date too — for example, to estimate when an already-born baby was conceived.
What is implantation and when does it happen?
Implantation is when the fertilised egg attaches to the uterine lining, typically 6–12 days after conception. It is when pregnancy hormone (hCG) production starts, which is why tests only turn positive several days after conception.
Can I use this to work out who the father is?
Only very cautiously. The fertile window spans about six days and ovulation timing varies cycle to cycle, so dates that are close together cannot be separated reliably. Only DNA testing can answer paternity questions definitively.
This tool provides general health information, not medical advice. Consult a healthcare professional before making decisions based on it. Dates are estimates — only an ultrasound can date a pregnancy accurately; always confirm with your healthcare provider.