How is the due date calculated?

Using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period, then adjust if your cycle is not 28 days.

How accurate is a due date?

It is an estimate, not a deadline. Only about 4% of babies arrive exactly on their due date, and most births happen within two weeks either side.

What are the three trimesters?

The first trimester runs to week 13, the second from week 14 to 27, and the third from week 28 until birth.

Pregnancy Due Date Calculator

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First day of last period

Cycle length (days)

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Estimate your due date from your last period, adjusted for your cycle length, and see your current week and trimester.

How to use Pregnancy Due Date Calculator

Open the toolNo signup and nothing to install; it works as soon as the page loads.
Set “First day of last period”
Set “Cycle length (days)”
Read the resultChange any input and the result is recalculated immediately.

Nothing you enter leaves your device

All of the work happens inside your browser. Whatever you type is never packaged up and sent to a server, and no request this page makes carries it.

That makes it safe for contract amounts, personal details and internal documents you would rather not hand to a third party — close the tab and it is gone.

What this tool does not do

Pregnancy Due Date Calculator only does what a browser can do on its own — it talks to no backend service. So it keeps no history for you and syncs nothing across devices; if you need a record, copy or download the result.

It is also not a substitute for professional judgement. For legal, medical or tax decisions, treat the output as a starting point rather than a final answer.

Frequently Asked Questions

How is the due date calculated?: Using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period, then adjust if your cycle is not 28 days.
How accurate is a due date?: It is an estimate, not a deadline. Only about 4% of babies arrive exactly on their due date, and most births happen within two weeks either side.
What are the three trimesters?: The first trimester runs to week 13, the second from week 14 to 27, and the third from week 28 until birth.