Pregnancy due date calculator

The calculator needs a single date: the first day of your last menstrual period. From there it applies Naegele’s rule, the method most commonly used to give a first estimate of a due date. It adds 280 days, which is 40 weeks, to the first day of your last period, and the date it lands on is your estimated due date.

It can seem strange that a pregnancy is counted from a period rather than from conception, but that is how pregnancy weeks are usually counted. The first day of your last period is a date most people can remember or look up, while the exact day of conception is rarely known. Counting from the period means that, by this convention, the first two weeks of a pregnancy fall before conception has happened.

The calculator also shows an estimated conception date, 14 days after the first day of your last period. That date comes from the same model the rule is built on: a 28-day cycle in which ovulation happens about 14 days before the next period would have started. Conception usually happens close to ovulation, so the calculator places it there.

Say your last period started on 1 March 2026. The calculator adds 280 days to 1 March and gives an estimated due date of 6 December 2026. It places the estimated conception date 14 days after the start of that period, on 15 March 2026.

Notice what the calculator does not ask for: your cycle length. Naegele’s rule assumes every cycle is 28 days long, so two people whose last periods started on the same day get exactly the same due date, whether their cycles usually last 24 days or 35. That simplicity is what makes the rule quick to use, and it is also the main reason the estimate can be off, as the next section explains.

The 280-day rule only lines up neatly with conception when ovulation happens around day 14 of the cycle. Many people have cycles that are longer or shorter than 28 days, and that changes when ovulation, and so conception, is likely to have happened.

Take the example above again, but with cycles that usually last 35 days. Using the same assumption that ovulation comes about 14 days before the next period, ovulation would fall around day 22, on 22 March rather than 15 March. Conception would then have happened about a week later than the calculator assumes, and the due date would move with it, to around 13 December instead of 6 December. With cycles shorter than 28 days, the shift goes the other way and the date moves earlier.

This calculator does not make that adjustment. It always applies the standard rule, so if your cycles are regularly much longer or shorter than 28 days, treat the result as a starting point rather than a fixed date. Your healthcare provider can take your usual cycle length into account.

The estimate is only as good as the date you enter and the assumptions behind the rule. It is less reliable if your cycles are irregular, because there is no typical day on which ovulation happens. It is also less reliable if you are not sure when your last period started, if you had bleeding or spotting in early pregnancy that you mistook for a period, or if you conceived soon after stopping hormonal contraception, before your cycles had settled into a regular pattern.

Even when everything fits the model, a due date is not a deadline. Only about 1 in 20 babies arrive on their exact due date. It is more useful to think of the result as the middle of the period in which your baby is likely to be born than as the day it will happen.

An early ultrasound gives your care provider a more precise date, and it is that date, not this calculator, that your prenatal care will be planned around. Use this tool to get a first idea of your timing, and confirm it with a healthcare professional.

The two weak points of any due date calculation are the starting date and the cycle length. Tracking your cycle fixes both. If you have logged your periods, you know the exact first day of your last one instead of guessing it, and you can see your real average cycle length rather than assuming 28 days.

That history is useful to bring to your first prenatal appointment. Knowing that your cycles usually last 33 days, for example, or that they vary a lot from month to month, helps your healthcare provider decide how much weight to give a date-based estimate. You can track your cycle over time with Evecare, and keep the record on hand for when you need it.