Free Pregnancy Tool
Ultrasound Gestation Period
Enter the date of your ultrasound and the gestational age it showed. We work out your due date and how far along you are today.
General information only, not medical advice. Ask your doctor or midwife.
How this works
Formula: Due date = scan date + (280 days − gestational age at scan). Today's age = age at scan + days since the scan.
- An ultrasound report gives a gestational age, such as 8 weeks 2 days, on the scan date.
- The calculator adds the days left to reach 280 days (40 weeks) to get the due date.
- If you also enter your last period, it compares the two due dates and shows the gap ACOG (the American College of Obstetricians and Gynecologists) uses for re-dating: more than 5 days before 9 weeks, 7 days from 9 to 15 weeks 6 days, 10 days from 16 to 21 weeks 6 days, 14 days from 22 to 27 weeks 6 days, and 21 days from 28 weeks.
Everything is worked out in your browser. Belly Weekly does not send or store your answers.
Questions
Which due date counts, ultrasound or period?
Your care team decides. Early ultrasounds are usually the most accurate for dating.
Why do later scans not change my due date?
Later in pregnancy babies vary more in size, so later scans are less useful for dating.
What if I had more than one early scan?
Care teams usually keep the date from the earliest good-quality scan.