How the due date calculator works
This tool estimates a pregnancy due date from the first day of your last menstrual period using the common method of adding 280 days, about 40 weeks, with a small adjustment if your cycle is longer or shorter than 28 days. It also estimates how many weeks along you are now and which trimester that falls in. These are general estimates only and not medical advice; your healthcare provider can confirm dates with an exam or ultrasound.
How the estimate is made
The standard approach counts 280 days from the first day of the last period, which assumes ovulation around day 14. If your typical cycle is longer or shorter, the estimate shifts by the difference, since ovulation timing moves with cycle length. An early ultrasound is generally the most accurate way to date a pregnancy.
Weeks and trimesters
Pregnancy is counted in weeks from the last period, so you are considered a couple of weeks along even at conception. The first trimester runs through about week 13, the second through about week 27, and the third from there to birth. Only about one in twenty babies arrive exactly on the estimated date, so treat it as a target window.
Important note
This calculator is for general information and planning only. It does not replace medical care, and it cannot account for individual circumstances. Always confirm dates and any health questions with a doctor or midwife, who can give guidance specific to you.
Frequently asked questions
How is a due date calculated? Commonly by adding 280 days to the first day of the last period, adjusted for cycle length.
How accurate is it? It is an estimate; an early ultrasound is more precise, and most births occur within a couple of weeks either side.
Is this medical advice? No. It is general information only; confirm with your healthcare provider.
Related calculators: Age, Date, Days Between Dates.