Due date by ultrasound

Because in the first trimester embryos grow at a remarkably uniform rate — the natural variation in size between two healthy pregnancies of the same age is tiny, so a single measurement pins the age down to within about five to seven days. Later in pregnancy babies diverge in size for reasons that have nothing to do with their age, so the same measurement gets less and less precise. That's why ACOG's Committee Opinion 700 calls first-trimester ultrasound the single most accurate dating method — better than a remembered period date, which also assumes a textbook 28-day cycle with day-14 ovulation.

The crown–rump length (CRL) is the embryo measured from the top of the head to the bottom of the torso, in millimetres — the standard first-trimester measurement. This tool converts it to gestational age with the Robinson & Fleming formula, published in 1975 and still the basis of most dating charts: gestational age in days = 8.052 × √(CRL × 1.037) + 23.73. Once the age on the scan day is known, the due date is simply the day the pregnancy reaches 280 days. The formula is validated for a CRL of 4 to 84 mm, roughly 6 to 14 weeks.

It depends on how big the gap is and how far along the scan was, and ACOG publishes the exact thresholds. At 8 weeks 6 days or earlier, the ultrasound takes over when it differs from your period-based date by more than 5 days; from 9 weeks to 15 weeks 6 days, more than 7 days; from 16 weeks, more than 10 days; from 22 weeks, more than 14 days; and from 28 weeks, more than 21 days. Inside the threshold, your period date stands. Enter your last period above and the tool applies the table for you — but the call on your chart belongs to your provider.

Generally no — and that's deliberate. Once a due date has been set by an accurate early ultrasound, ACOG's guidance is that later scans should rarely change it, because dating only gets less precise as pregnancy goes on: a third-trimester scan is uncertain by three weeks or more. A baby measuring 'ahead' or 'behind' at a 32-week growth scan isn't evidence the due date is wrong — it's information about growth, which your provider interprets against the fixed date. The earliest reliable scan is the due date of record for the whole pregnancy.

Because that's the range the Robinson–Fleming formula was built and validated on — roughly 6 to 14 weeks of pregnancy. Below about 4 mm the embryo is too small to measure reliably, and dating usually leans on gestational-sac findings instead. Above 84 mm (around 14 weeks) the fetus starts to curl and flex, so a crown-to-rump line stops tracking age; sonographers switch to other measurements — head circumference, biparietal diameter, abdominal circumference, and femur length. If your report is from 14 weeks or later, use the 'weeks + days on the report' mode instead of the CRL.

Yes. This calculator runs entirely in your browser: the scan date, CRL, and period date you enter are computed on your own device and are never sent to a server, stored, or shared — close the tab and everything is gone. We treat reproductive data as sensitive by default, so there is nothing here to subpoena, sell, or leak. It's an educational estimate, not medical advice or a diagnosis: the gestational age and due date on your sonographer's report, as read by your provider, are the numbers of record.