Two ways to estimate conception
You can work toward the conception date from either end of a pregnancy. If you know the first day of your last menstrual period (LMP), conception is estimated as about 14 days later, since ovulation usually falls near the middle of the cycle.
If instead you already have a due date — for example from an ultrasound — conception can be estimated by counting back 266 days, which is 280 days of gestation minus the roughly two weeks between the LMP and ovulation. Pick whichever reference you have and the tool applies the matching formula.
Why the result is a window, not a single day
The exact moment of fertilisation is almost never known, so the calculator shows a fertile window of several days around the central estimate rather than pretending to pinpoint one day.
The window leans toward the days before ovulation because sperm can survive for up to about five days, while the egg itself lasts only around a day after release. Conception is most likely on the day of ovulation and the day or two before it.
When this estimate may be off
The LMP method assumes ovulation around day 14 of a regular cycle, which does not hold for everyone. A few situations widen the margin of error:
- Cycles that are noticeably longer, shorter, or irregular.
- An uncertain or poorly remembered last-period date.
- A due date that was itself estimated rather than confirmed by ultrasound.
- Recent use of hormonal contraception, which can disrupt ovulation timing.
Medical disclaimer
This calculator gives general estimates for educational purposes only and is not medical advice. It cannot confirm when conception occurred or establish paternity or legal dates. For accurate dating, consult a doctor or qualified healthcare provider.
Formula
From LMP: conception = LMP + 14 days; From due date: conception = dueDate − 266 daysFrequently asked questions
- Why is conception about two weeks after the period starts?
- Ovulation typically occurs near the middle of a cycle, around 14 days after the LMP, which is when conception can happen.