Gestational Age by Ultrasound: The Redating Table

An ultrasound reports gestational age on the day of the exam. To get today’s, you add the days that have gone by since. Enter the scan date and the weeks and days it reported.

What the sonographer actually measures

In the first trimester the measurement is crown-rump length, taken in a true midsagittal plane, and the number used for dating should be the mean of three discrete measurements. Through 13 weeks and 6 days that gives an accuracy of plus or minus 5 to 7 days, and it is tighter the earlier the scan is done.

Beyond 84 mm — roughly 14 0/7 weeks — crown-rump length loses precision, and dating switches to a composite of biparietal diameter, head circumference, abdominal circumference and femur length. Gestational sac diameter is not used for dating at all.

Precision degrades steadily from there. ACOG puts biometry between 14 0/7 and 21 6/7 weeks at plus or minus 7 to 10 days, between 22 0/7 and 27 6/7 weeks at plus or minus 10 to 14 days, and anything from 28 0/7 weeks onward at plus or minus 21 to 30 days.

The redating table

This is the part clinicians look up. ACOG deliberately chose single-point cutoffs so that institutions would date pregnancies the same way. If the disagreement between the scan and the last-period date exceeds the value in the third column, the due date moves to the scan.

Gestational age at the scan	Dating method	Change the date if the gap exceeds
Up to 8 weeks 6 days	crown-rump length	5 days
9 0/7 to 13 weeks 6 days	crown-rump length	7 days
14 0/7 to 15 weeks 6 days	biometry	7 days
16 0/7 to 21 weeks 6 days	biometry	10 days
22 0/7 to 27 weeks 6 days	biometry	14 days
28 0/7 weeks and beyond	biometry	21 days

Read the column downward and the logic is visible: the later the first scan, the more slack the guideline allows, because the measurement itself has more slack. It is not that a late scan is trusted more — it is trusted less, so it takes a bigger disagreement to act on it.

Worked example

Scan on April 1 reading 9 weeks and 0 days, today May 6:

• Days since the exam: 35, which is 5 weeks
• Gestational age today: 14 weeks and 0 days
• Trimester: second

If that April scan had disagreed with the last-period date by 6 days, the rule above would have moved the due date to the scan, since the exam fell in the 9 0/7 to 13 6/7 window.

Dated once, then left alone

The strong recommendation is not that ultrasound always wins — it is that a pregnancy gets dated early and then stops moving. Once a first-trimester scan sets the date, a second-trimester scan should not revise it. Later changes are reserved for rare circumstances, discussed with the patient and written into the record, because the date drives delivery planning.

And there is a deadline: a pregnancy without a scan confirming or revising the date before 22 0/7 weeks is classified as suboptimally dated.

Compare with the method this one corrects, gestational age by LMP, and see all three approaches side by side in the gestational age calculator. Twins are dated by the same measurement but a different selection rule — that is on twin gestational age.

This content is informational. Only your prenatal care team assigns the gestational age of record.