Estimate when conception is likely to have happened. Work it back from a known due date, or forward from the first day of your last period and your cycle length.
Choose a method and enter your dates.
This tool gives an estimate only and is not medical advice. Conception timing cannot be known exactly. Confirm any dates with a healthcare provider.
Conception timing is an estimate. The window allows for the days around ovulation when conception is possible.
About this calculator
Conception happens when a sperm fertilises an egg, at or just after ovulation. There is no way to know the exact moment, so this is always an estimate. The calculator offers two routes. If you know your estimated due date, it counts back 266 days, the average length of pregnancy measured from conception, to find the likely conception date. If you know the first day of your last period, it places conception at ovulation, about 14 days before the next period would have been due, adjusted for your cycle length.
The result comes with a window of several days rather than a single date, because conception can follow intercourse that happened earlier. Sperm can survive in the body for up to about five days, so the fertilising sperm may have been present well before the egg was released. Pregnancy is normally dated from the first day of the last period, not from conception, which is why a pregnancy is counted as roughly two weeks further along than the time since conception. The American College of Obstetricians and Gynecologists and the NHS explain how pregnancy dating works.
Frequently asked questions
How is the conception date calculated?
There are two common ways, and this calculator offers both. From a due date, conception is estimated as 266 days earlier, since that is the average length of pregnancy counted from conception. From the first day of the last period, conception is placed at ovulation, about 14 days before the next period was due, adjusted for cycle length. Both are estimates. The true moment of conception cannot be pinned down exactly, which is why the result includes a window of several days.
Why is conception about two weeks after my last period?
Pregnancy is dated from the first day of the last menstrual period, but conception does not happen until ovulation, which is roughly two weeks later in a typical cycle. So at the moment of conception a pregnancy is already counted as about two weeks along. This is a long-standing medical convention, used because the last period is a date most people can identify, while the exact day of conception usually cannot be known.
How accurate is a conception date calculator?
It gives a reasonable estimate, not an exact answer. Conception cannot be observed, and the calculation rests on averages: a 266-day pregnancy from conception, and ovulation around 14 days before the next period. Real cycles vary, and ovulation can happen earlier or later than expected. The window the calculator shows reflects that uncertainty. An early dating ultrasound is the most reliable way to estimate pregnancy timing, and a healthcare provider can confirm it.
Can a conception calculator tell me who the father is?
Not with certainty. The calculator estimates a likely conception date and a window of several days around it, but it cannot narrow timing down to a single day, and it cannot account for sperm surviving for up to about five days after intercourse. Questions of paternity cannot be answered by date arithmetic. A DNA paternity test is the only reliable way to establish parentage, and such tests are widely available.
Is the conception date the same as the ovulation date?
They are very close. Conception, the fertilisation of the egg, happens at or within a day of ovulation, because the egg is viable for only about a day after it is released. So for practical purposes the calculator treats the conception date and the ovulation date as the same when working from a last period. Any difference is a matter of hours rather than days, far smaller than the overall uncertainty in the estimate.