Dating a pregnancy by ultrasound
An ultrasound measures the size of the embryo or fetus and converts it into a gestational age — how many weeks and days pregnant you are on the day of the scan. Because it measures the pregnancy directly, a scan doesn’t depend on remembering your last period or on when you ovulated. This calculator takes that measurement and the scan date and works out your due date.
Due date = scan date + (280 days − gestational age on the scan)
Example: a scan on September 1, 2026 measuring 9 weeks 2 days (65 days) gives a due date 215 days later: April 4, 2027.
What the scan measures
| Stage | Main measurement | Dating accuracy |
|---|---|---|
| Up to ~14 weeks | Crown–rump length (CRL) | Best — about ±5–7 days |
| 14–22 weeks | Head circumference, femur length and others | Good — about ±7–10 days |
| After 22 weeks | Several measurements combined | Less precise for dating — about ±2–3 weeks |
Finding the numbers on your report
Scan reports usually state the gestational age as “GA 9w2d”, “9+2” or “9 weeks 2 days”, sometimes alongside an EDD. Enter the scan date — not today’s date — and the weeks and days shown. The calculator then shows how far along you are today and your key dates.
Scan vs period dates
If your scan date and your period date disagree, see the EDD Calculator Pregnancy, which applies the redating rule clinicians use. Once your due date is confirmed by an early scan, it isn’t normally changed by later scans.