Conception Calculator
When did you conceive?
Results are estimates only. Consult your healthcare provider.
How conception timing works
Conception typically occurs about 2 weeks after the first day of the last menstrual period, during ovulation. The fertilized egg then takes 6–12 days to implant in the uterine wall.
Since sperm can survive 3–5 days and the egg is viable for about 24 hours, the conception window spans roughly 6 days. This calculator provides the most likely conception date based on a 40-week pregnancy.
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How does the conception calculator work?
Working backwards from a due date to estimate when conception occurred is possible because pregnancy follows a predictable timeline. If your estimated due date is known, subtracting 266 days (38 weeks from the moment of conception, or 40 weeks from LMP) gives an approximate conception date. If you are using your last period date, our calculator adds the typical ovulation lag — usually 14 days into a 28-day cycle — to estimate when the egg was likely fertilised. The result is a conception window rather than a single day, because sperm can survive up to five days and the egg is only viable for 24 hours.
Why is the conception date a range, not a single day?
Fertilisation depends on both sperm survival and egg release. Sperm deposited up to five days before ovulation can still successfully fertilise an egg when it is released. This means the "conception" you see on your calculator is better understood as a window — often spanning several days — during which intercourse could have resulted in pregnancy. Pinpointing an exact day of conception is biologically impossible without real-time ultrasound monitoring of follicle development and egg release, which is only done in clinical fertility settings.
Does conception date affect the due date?
The due date is typically calculated from the LMP rather than the conception date, precisely because the LMP is easier to know. However, if you know your ovulation date precisely (via OPK or ultrasound), your healthcare provider may use it to calculate a more accurate EDD. In IVF pregnancies, where the date of egg retrieval and embryo transfer are known exactly, dates are calculated from the egg retrieval date, giving a much more precise gestational timeline.
Paternity and legal contexts
The conception calculator is sometimes used in legal or paternity contexts to estimate when intercourse may have resulted in pregnancy. While this calculator gives an estimate based on standard biological timelines, it is important to understand that its margin of error spans several days to a week. For legal or medical certainty, DNA testing or detailed clinical records are the only reliable sources. This tool does not provide legally admissible evidence of any kind.
Early signs that conception has occurred
Most people do not experience any symptoms in the first week after fertilisation. Implantation — when the fertilised egg attaches to the uterine wall — typically occurs 6–12 days after ovulation and may cause light spotting or mild cramping, though many people notice nothing at all. The earliest reliable sign is a positive pregnancy test, which can appear as early as 10–14 days after conception. Home pregnancy tests detect hCG (human chorionic gonadotropin), the hormone produced by the developing placenta immediately after implantation.