The ACOG CO 700 thresholds
The window is chosen by the ultrasound gestational age. Redate only when the difference exceeds the threshold — exactly at the threshold, LMP dating stands.
| Ultrasound GA | Redate if difference is more than |
|---|---|
| up to 8w6d | 5 days |
| 9w0d – 15w6d | 7 days |
| 16w0d – 21w6d | 10 days |
| 22w0d – 27w6d | 14 days |
| 28w0d and beyond | 21 days |
OB wheel for clinicians with ACOG redating, IVF transfer dating and the Bishop score. No account, no patient data stored, works offline.
Get Partus on the App StoreCommon questions
Why did the due date change after an ultrasound?
Early ultrasound measures the embryo directly and is accurate to within days, while LMP dating relies on recall and an assumed 28-day cycle. When the two disagree by more than the ACOG threshold for that gestational age, the ultrasound date is adopted.
Does redating work in both directions?
Yes. The rule applies whether the ultrasound is behind or ahead of the LMP dates — only the size of the difference matters.
Why do the thresholds grow with gestational age?
First-trimester crown-rump length dates a pregnancy to within about 5–7 days; later in pregnancy fetal size varies more between individuals, so a larger discrepancy is needed before the calendar is overruled.
What if the LMP is unknown or unreliable?
Ultrasound dating is used from the start — there is nothing to redate against.
Source
Methods for Estimating the Due Date. ACOG Committee Opinion No. 700, May 2017. This page is a reference tool for healthcare professionals. It does not provide medical advice or diagnosis.