Pregnancy Due Date Calculator

An estimated due date is a midpoint, not an appointment. This calculator applies Naegele's rule to the first day of your last menstrual period, adjusts for cycle length, and reports the date along with how far along you currently are and where the trimester boundaries fall.

Naegele's rule, adjusted for your cycle

The rule is attributed to the German obstetrician Franz Naegele in the early nineteenth century and has barely changed since:

estimated due date = first day of last period + 280 days + (cycle length - 28)
estimated conception = first day of last period + (cycle length - 14)

Two hundred and eighty days is forty weeks, which is why pregnancy is described as forty weeks despite conception occurring roughly a fortnight after the period begins. The count starts from a day you can observe rather than one you usually cannot.

Take a last period beginning 5 January 2026. On a textbook 28-day cycle that gives 12 October 2026. On a 32-day cycle, ovulation happens about four days later, so the rule shifts by four days to 16 October 2026, with conception estimated around 23 January. The adjustment matters most for people with long or short cycles, and it assumes the luteal phase is a fixed fourteen days — which is roughly true on average and not reliably true for any individual.

Two corrections are worth naming. Dates entered in the future are now rejected: the previous code took an absolute difference and told someone whose period was four months away that they were eighteen weeks pregnant. And dates are now built in your device's local time zone, so users west of UTC no longer see a due date one day early.

Almost nobody delivers on the date

NHS guidance notes that only about 1 in 20 babies arrive on their due date. The estimate is the centre of a distribution roughly five weeks wide, and the great majority of spontaneous births fall within a fortnight either side of it. Treating the date as a deadline sets up an entirely avoidable stretch of anxiety in the final weeks.

A first-trimester dating scan is more accurate than any calculation from dates, because early fetal growth is highly consistent between pregnancies. Where a scan and a menstrual-dates estimate disagree by more than about a week, obstetric practice is to date the pregnancy by the scan, and that revised date supersedes anything on this page.

What "term" means, precisely

The definitions below come from the joint ACOG, SMFM and NICHD workgroup that replaced the single "term" label with four categories, because outcomes differ measurably across them:

The change was made specifically to discourage elective delivery in the 37 and 38 week window on the grounds that it was "term" and therefore equivalent. It is not equivalent. Timing decisions in the late-term and post-term range belong to your midwife or obstetrician, who will weigh monitoring against induction on the basis of your pregnancy rather than a calendar.

Where the calculation does not apply

It assumes regular cycles, a single pregnancy, and ovulation at a predictable point. Twins and higher multiples are dated and delivered on a different schedule. Conception after fertility treatment is dated from the transfer or retrieval, not from a period. Irregular cycles, recent hormonal contraception, or bleeding in early pregnancy all undermine the starting point the rule depends on.

For measurements unrelated to dating, the wider health tools section covers weight, energy and hydration, though note that the BMI calculator is not meaningful during pregnancy.

Your dates are processed in the browser and never transmitted; your own browser may retain them so a reload does not clear the form. No clinician has reviewed this page, and it is not a substitute for antenatal care.