What is a Pregnancy Week Calculator?
A Pregnancy Week Calculator tells you how many weeks and days pregnant you are, which trimester you are in and your estimated due date, from the first day of your last period.
Pregnancy is counted from the first day of the last period, about two weeks before conception. The first trimester runs to week 13, the second to week 27 and the third until birth.
How to use the Pregnancy Week Calculator
- Enter first day of your last period.
- Tap Calculate to see the result.
- Tap Reset to clear the fields and try again.
Who should use it
- Expecting parents
- Family members following a pregnancy
- Anyone tracking milestones
Key terms
- Trimester
- One of the three stages of pregnancy.
- Gestational age
- Weeks of pregnancy counted from the last period.
Frequently asked questions
Why is pregnancy counted from my last period?
It is a fixed date most people know, and doctors use it as the standard.
Is the due date exact?
No. Most babies arrive within two weeks either side.
Is this medical advice?
No. Consult a doctor.
Is my data stored?
No. Everything you enter stays in your browser.
Last updated: 1 November 2026