Ultrasound Due Date Calculator: Scan Age or CRL

From scan age to due date

A scan report gives gestational age on the day of the scan, such as 12 weeks 3 days. The due date is the date when that pregnancy reaches 40 weeks 0 days: the scan date plus 280 days, minus the age at the scan.¹

Worked example: 12 weeks 3 days on the scan

August 20, 2026Scan date
+280 days40 weeks
−87 days12 weeks 3 days already reached
=Monday, March 1, 2027Estimated due date

Ultrasound in the first trimester, up to 13 weeks 6 days, is the most accurate way to establish or confirm gestational age, to within about 5 to 7 days.¹ Later scans are less precise for dating because babies grow at different rates.

Converting a crown-rump length

If your report gives a crown-rump length (CRL) but no age, the calculator converts it with the Robinson and Fleming equation, as encoded in the LOINC standard: gestational age in days = 8.052 × √CRL(mm) + 23.73.^2,3 We round to the nearest whole day and accept 6 to 84 mm. Above 84 mm, ACOG recommends dating with other measurements.¹

For comparison we also show the Hadlock (1992) equation, which uses centimetres inside an exponential: age in weeks = exp(1.684969 + 0.315646c − 0.049306c² + 0.004057c³ − 0.000120456c⁴), where c is CRL in cm.^4,5 Ultrasound machines may use either equation or another one, so a day’s difference from your report is normal.

Crown-rump length and gestational age by equation
CRL	Robinson	Hadlock
6 mm	6+1	6+3
10 mm	7+0	7+0
15 mm	7+6	7+6
20 mm	8+4	8+4
30 mm	9+5	9+6
45 mm	11+1	11+2
60 mm	12+2	12+3
84 mm	14+0	14+2

Measuring CRL correctly needs a trained sonographer and the right image. Use the gestational age on your report if it has one, and don’t try to measure from a printout.⁶

When a scan changes the due date

ACOG, AIUM and SMFM recommend changing a period-based due date to the ultrasound date when the two differ by more than a set number of days. The allowance grows as pregnancy goes on, because scan dating becomes less precise.¹

Discrepancy that supports redating (ACOG Committee Opinion 700)
Gestational age by last period, at the scan	Change the date if the difference is more than
Through 8 weeks 6 days	5 days
9 weeks 0 days to 13 weeks 6 days	7 days
14 weeks 0 days to 15 weeks 6 days	7 days
16 weeks 0 days to 21 weeks 6 days	10 days
22 weeks 0 days to 27 weeks 6 days	14 days
28 weeks 0 days and beyond	21 days

“More than” matters: a difference of exactly 7 days at 10 weeks does not cross the line. Clinicians also weigh how certain the period date is. For IVF pregnancies, the IVF date is used instead.¹

Why later scans differ

Once a due date has been set from an early scan, it should rarely change. Later scans measure size, and a baby measuring small or large is information about growth rather than a reason to move the date. Redating late in pregnancy can hide growth restriction.¹ Read more in ultrasound vs last period.

Questions people ask

Why does my scan say a different due date from my period?

Ovulation may have happened earlier or later than the calculation assumes, or the period date may be uncertain. Small differences are expected; your care team changes the date only when the difference passes thresholds that depend on how far along you are.¹

My later scans keep giving different dates. Should my due date change?

Usually not. Once a due date is set from an early scan, later scans measure growth rather than age, and changing the date to match them could hide a growth problem.¹

What does CRL mean on my scan report?

Crown-rump length: the length of the embryo or fetus from the top of the head to the bottom, measured in the first trimester. It is the most accurate way to estimate gestational age up to 13 weeks 6 days.¹

Ultrasound due date calculator