Due Date by Ultrasound: the ACOG Redating Rules

An ultrasound in the first trimester is the most accurate way to date a pregnancy. Enter the date of the exam and the gestational age it reported, in weeks and days, and the calculator returns the due date it implies.

How the calculator works backward

The scan does not measure a due date. It measures how far along the pregnancy is on the day of the exam. So the arithmetic runs in reverse: subtract the measured gestational age from the exam date to recover the equivalent first day of the last period, then add 280 days. The result is anchored in the embryo, not in anyone’s memory of a calendar.

Worked example. A scan on April 1 reports 12 weeks 0 days.

• 12 weeks is 84 days, so the equivalent last period began on January 7.
• January 7 plus 280 days gives a due date of October 14.

What the machine actually measured, before 14 weeks, is the crown-rump length: the straight-line distance from the top of the head to the caudal rump, taken in a true midsagittal plane and averaged over three measurements. Past 84 mm — around 14 weeks 0 days — that measurement loses accuracy and the report switches to head circumference, biparietal diameter, abdominal circumference and femur length.

The gap that changes the date

Any scan will disagree slightly with the period-based date. ACOG converts that judgment call into a single threshold per window: below it, the last-period date stands; above it, the scan date becomes the due date.

Gestational age at the scan	Ultrasound accuracy	Redate if the gap is
Before 9 weeks 0 days	about 5 to 7 days	more than 5 days
9 weeks 0 days to 13 weeks 6 days	about 5 to 7 days	more than 7 days
14 weeks 0 days to 15 weeks 6 days	about 7 to 10 days	more than 7 days
16 weeks 0 days to 21 weeks 6 days	about 7 to 10 days	more than 10 days
22 weeks 0 days to 27 weeks 6 days	about 10 to 14 days	more than 14 days
28 weeks 0 days and beyond	about 21 to 30 days	more than 21 days

Read the two right-hand columns together. The threshold is roughly the size of the scan’s own error bar, which is the point: a difference smaller than the measurement’s uncertainty is not evidence that the period date was wrong.

The first good scan is the one that counts

ACOG’s rule after that is conservative on purpose. Once dating is established, “subsequent changes to the EDD should be reserved for rare circumstances.” With rare exception, a second-trimester scan does not adjust a date already set in the first trimester; from then on scans are measuring growth against the established date, and a fetus that measures small is treated as a growth question rather than a calendar question.

Two deadlines are worth knowing. A pregnancy with no ultrasound confirming or revising the date before 22 weeks 0 days is classified as suboptimally dated. And an IVF pregnancy is not redated by ultrasound at all, because the conception date is already known — see the IVF due date calculator.

If your scan has not happened yet, start from due date by last period and expect the number to be checked.

This content is informational and does not replace prenatal care. Only the clinician reading your scan can assign or revise the due date in your record.