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Due Date Calculator Guide

A due date calculator gives an estimated due date from the first day of the last menstrual period and your cycle length. This guide explains how that estimate is produced, when ultrasound can change the dating, and why the result should be used as a planning window rather than a promise of the exact birth day.

Who this guide is for Expecting parents who want a clear explanation of estimated due date, gestational age, and how scan dating fits into the picture.
Best companion tool Due Date Calculator
Pregnancy due date illustration with calendar and timing wheel
The estimated due date is a clinical planning anchor, not a guarantee that labor will begin on one exact day.

Most due date calculators begin with the first day of the last menstrual period. From there, pregnancy is estimated at around 280 days, or 40 weeks. If you know your cycle is shorter or longer than the common 28-day assumption, adjusting cycle length can produce a more realistic early estimate.

Why the estimate can change

Last-period dating is useful, but it depends on remembering the date correctly and having a fairly predictable cycle. An early ultrasound can sometimes confirm or revise the estimate because first-trimester dating is generally more precise than relying on memory alone.

Three-trimester pregnancy timeline illustration
Pregnancy planning works better when you think in trimester milestones, not just the final due date.
Key timing idea Once a due date is chosen clinically, later ultrasounds usually do not keep changing it unless there is a strong medical reason.

Worked example

If the first day of the last period was January 1 and the cycle is close to 28 days, a due date calculator will estimate a date around 40 weeks later. If the cycle is usually longer, ovulation may have happened later, which can push the likely due date forward slightly.

Further reading

NHS Due Date Calculator Patient-friendly guidance on estimated due date, cycle length, and what happens next in pregnancy care. ACOG: Methods for Estimating the Due Date Clinical guidance on how last menstrual period, ultrasound, and assisted reproduction affect official dating.

FAQ

Does every pregnancy last exactly 40 weeks
No. Forty weeks is the standard estimate from the last menstrual period, but normal full-term delivery can happen within a wider range.
Can an ultrasound change the due date
Yes. Early ultrasound dating can confirm or revise the estimated due date when it does not match the menstrual-date estimate closely enough.
Should I worry if the baby is not born on the estimated due date
Not by itself. The date is a planning estimate, and many healthy pregnancies deliver before or after that exact day.
Pregnancy tool

Use the calculator for a first estimate, then compare it with medical dating information as your pregnancy care progresses.

Try the Due Date Calculator