Pregnancy Due Date Calculator

An estimated due date marks the middle of a window of likely dates, and very few babies keep to it. This calculator applies Naegele's rule to the first day of your last menstrual period, adjusts for cycle length, and shows the date along with how far along you are now 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 it 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. That's why pregnancy is described as forty weeks even though conception happens roughly a fortnight after the period begins: the count starts from a day you can observe, since the day of conception usually can't be.

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 if your cycles are long or short. It also assumes the luteal phase is a fixed fourteen days, which is roughly true on average but not reliably true for any one person.

Two earlier bugs are now fixed. Dates in the future are rejected. The previous code took an absolute difference, and told someone whose period was four months away that they were eighteen weeks pregnant. Dates are also built in your device's local time zone now, so if you're west of UTC you 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. If you treat the date as a deadline, you're setting yourself up for 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. When 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 overrides 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, which it isn't. Timing decisions in the late-term and post-term range belong to your midwife or obstetrician. They'll weigh monitoring against induction based on your own pregnancy, not on a calendar.

Where the calculation doesn't 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. After fertility treatment, conception is dated from the transfer or retrieval instead of from a period. Irregular cycles, recent hormonal contraception, or bleeding in early pregnancy all undermine the starting point the rule depends on.

For measurements that aren't about dating, the wider health tools section covers weight, energy and hydration. Note, though, that the BMI calculator isn't meaningful during pregnancy.

Your dates are processed in the browser and never sent anywhere, though your own browser may keep them so a reload doesn't clear the form. No clinician has reviewed this page, and it is not a substitute for antenatal care.