How an ultrasound due date is calculated
A dating scan measures the size of the embryo or fetus and converts that measurement to a gestational age in weeks and days. The calculator then works out how many days of a 280-day pregnancy are left: due date = scan date + (280 - gestational age in days).
For example, if your scan shows 8 weeks 0 days (56 days), the remaining days are 280 - 56 = 224, so your due date is 224 days after the scan date.
When the dating scan wins
Period-based dating assumes a textbook 28-day cycle with ovulation on day 14. If your cycles are irregular, long, or short - or if you are unsure of your last period - the scan result is a more reliable anchor. Clinicians typically use the scan date if it disagrees with the period date by more than 5-7 days in the first trimester.
If you want to compare against a period-based estimate, try the last-period due date calculator. Once you have your due date, you can find out your current gestational week with the pregnancy week calculator.
Frequently asked questions
How does ultrasound dating work?
A sonographer measures the baby (in early pregnancy, the crown-rump length) and converts it to a gestational age in weeks and days. We add the remainder of a 280-day pregnancy to your scan date: due date = scan date + (280 - gestational age in days).
Is ultrasound or last period more accurate?
An ultrasound in the first trimester (roughly 8 to 13 weeks) is the most accurate dating method and is often used to override a period-based date, especially with irregular cycles or an uncertain last period. Later scans are less precise for dating.
Why did my due date change after a scan?
If the scan's gestational age differs from your period-based estimate by more than about a week, clinicians usually adopt the scan date. This is normal and means your dates were refined, not that anything is wrong.
These calculators give estimates based on cycle averages and standard formulas. They are for general information only and are not medical advice. For anything concerning your health or pregnancy, talk to a qualified healthcare provider.