Due Date by Ultrasound

Enter the date of a scan and the gestational age measured at it, and this page works out the due date the same way a sonographer's report does.

The method is simple: a pregnancy is counted as 280 days, so the due date is the scan date plus however many days remained at the scan. Early scans date pregnancies this way because embryo size varies very little in the first trimester.

Date of the ultrasound scan

Gestational age at scan — weeks

Gestational age at scan — extra days

Estimated due date

Days remaining at scan

Implied last period (day 1)

Estimated conception

Please read: This calculator gives estimates only — it is not medical advice and cannot diagnose or date a pregnancy with certainty. Only around 5% of babies arrive on their estimated due date, and most arrive within a week or so either side. Ultrasound dating performed by a clinician takes precedence over any calculator result. Always confirm your dates, screening windows and care plan with your healthcare provider. All dates are processed locally in your browser; nothing you enter is uploaded or stored.

Why providers trust early scans

In the first trimester, embryos grow at a remarkably consistent rate, so a crown–rump length measurement can date a pregnancy to within a few days. Later in pregnancy, babies' sizes diverge naturally — a big baby is not necessarily an older baby — so dating scans are done early, and the date from one is usually kept even if later scans suggest a different size trajectory.

If the due date your provider gave you differs from a last-period estimate, it is normally the scan-based date that stands. This calculator reproduces the arithmetic; it cannot reproduce the measurement, and it never overrides your clinician.