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Calculator

Pregnancy calculator

Estimate pregnancy week, trimester and due date from the first day of your last period.

Expectant parent looking at a calendar for a pregnancy calculation

This calculator uses the common clinical method that counts from the first day of the last menstrual period. You can see an estimated week, trimester and approximate due date on one screen.

Calculate

Result

Enter the details and press Calculate. The result will appear here.

How to use it

  1. Enter the first day of your last menstrual bleed.
  2. The calculation uses 280 days (40 weeks) from that date.
  3. The result shows today’s estimated week + days, trimester and due date.

Points to watch

  • Results can shift with irregular cycles or delayed ovulation.
  • An ultrasound date from your clinician may differ; clinical follow-up comes first.
  • The tool does not store records; the calculation runs in your browser.

Frequently asked questions

Why is pregnancy week counted from the last period?

The exact day of fertilisation is often unknown. The first day of the last period is easier to remember, so clinical gestational age usually starts there.

Is the result an exact birth date?

No. It is an estimate. Timing of birth varies; your clinician sets the follow-up plan.

If this tool differs from my doctor’s week, which is right?

Early ultrasound and clinical assessment come first. This tool is general information.

Source and method note

Counting clinical gestational age from the first day of the last period over about 280 days follows a common antenatal dating frame. Results do not replace a personal medical decision.

Related guides

Other tools

Due-date calculator Pregnancy-week calculator Ovulation calculator Baby-age calculator Pregnancy weight-gain calculator