Pregnancy Due Date Calculator

This pregnancy due date calculator estimates your due date from your last menstrual period and cycle length. A due date is counted from the first day of your last menstrual period, not from conception, because that date is the one most people can recall. Enter it here and this calculator estimates your due date, tells you how far along you are, and lays out the trimester milestones.

Naegele's rule

due date = LMP + 280 days
adjustment = (cycle length − 28) days

The standard rule adds 280 days (40 weeks). If your cycle is longer or shorter than 28 days, ovulation — and therefore the due date — shifts by the same difference, which this calculator applies.

What the dates mean

Pregnancy runs about 40 weeks: the first trimester to week 13, the second to week 27 and the third to birth. Only about 5% of babies arrive on the exact due date; anywhere from week 37 to 42 is considered full term.

Worked example

A last period beginning on 1 March 2026 with a 28-day cycle:

due date        = 1 Mar 2026 + 280 days = 6 Dec 2026
likely conception = 1 Mar 2026 + 14 days = 15 Mar 2026
2nd trimester   = +91 days  = 31 May 2026  (week 14)
3rd trimester   = +189 days = 6 Sep 2026   (week 28)

With a 30-day cycle the adjustment is +2, so every date shifts two days later and the due date becomes 8 December. With a 26-day cycle it shifts two days earlier. The offset applies to the due date and the conception estimate alike, because both move with ovulation.

Why it counts from your period and not from conception

Because the first day of a last period is a date most people can actually recall, while conception is invisible. Ovulation typically happens about 14 days into a 28-day cycle, so the first two weeks of a counted pregnancy are weeks in which you were not pregnant at all. That is not an error in the convention — it is the convention, and every clinician, scan report and pregnancy app uses the same one, which is why the weeks quoted to you will match what this tool shows.

The cycle adjustment exists precisely because that 14 days is not fixed. In a longer cycle ovulation comes later, so conception comes later, so the due date comes later. Adding a flat 280 days to every cycle length would systematically misdate anyone whose cycle differs from the average.

What this estimate cannot do

It assumes a regular cycle and a known last period, and it is an arithmetic projection rather than a measurement. Only about 5% of babies arrive on the exact due date; anything from week 37 to week 42 counts as full term, so the useful reading of the result is a five-week window with a midpoint rather than a single day.

An early ultrasound is more accurate than any date-based calculation because it measures the pregnancy directly — typically to within about a week in the first trimester, and less precisely later on. Where a scan date and a period date disagree, clinicians generally follow the early scan. Irregular cycles, uncertain dates, recently discontinued hormonal contraception, and IVF (where the transfer date is known exactly and the calculation differs again) all reduce what this page can tell you. Treat it as an orientation and confirm with your midwife or doctor. Age Calculator is the natural next step once there is a birthday to count from.

Private and precise

This calculator produces estimates from published formulas and is not medical advice. For anything about your health, talk to a doctor or a registered dietitian.