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Pregnancy Calculator

Estimate a due date and current gestational age from the last menstrual period, adjusted for cycle length.

Details

A cycle longer than 28 days pushes the due date later.

Your Results

What Pregnancy Calculator does

The standard estimate adds 280 days to the first day of the last menstrual period. That is Naegele's rule, and it assumes ovulation on day 14 of a 28-day cycle.

Because cycles vary, the estimate is adjusted here for the cycle length you enter, which shifts the date for anyone whose cycle is not 28 days.

  • Due date from the last menstrual period
  • Adjustment for cycle length
  • Current gestational age in weeks and days
  • Days remaining

How to use Pregnancy Calculator

  1. 1

    Enter the first day of your last period

    The first day of bleeding, not the last.

  2. 2

    Enter your typical cycle length

    A longer cycle means later ovulation and a later due date; a shorter cycle the reverse.

  3. 3

    Read the estimate

    Treat it as the centre of a range rather than an appointment.

Limits and known behaviour

  • This is an estimate. Only about one birth in twenty occurs on the estimated date, and full term spans several weeks either side.
  • An early ultrasound is more accurate than any date-based calculation, and clinicians will revise the date if the two disagree materially.
  • The calculation assumes a regular cycle. With irregular cycles the estimate is much less reliable.
  • It does not apply to IVF pregnancies, where the transfer date gives a more precise starting point.
  • Twins and other multiples typically deliver earlier than the estimate suggests.
  • This is an estimate from a published formula, not a medical assessment. It cannot diagnose anything, and it is no substitute for advice from a clinician who knows your history.

Privacy and data handling

Runs entirely in your browser

  • Every figure you enter stays in this page. The dates you enter is not transmitted, logged or stored anywhere.
  • There is no account, no saved history and no autosave: reloading the page clears the form.

Site-wide data handling, including analytics and advertising, is described in the privacy policy.

Frequently asked questions

How accurate is a due date?

Around 4% of births occur on the estimated date. Most occur within a week or two either side, which is why clinicians treat it as the middle of a window rather than a prediction.

Why does my clinic's date differ from this?

Because they will have used an early ultrasound measurement, which is more accurate than a date-based calculation and takes precedence when the two disagree.

Does this confirm a pregnancy?

No. It calculates dates from what you enter. Only a test and a clinical assessment can confirm a pregnancy.