Citation, DOI, disclosures and article data
Citation:
Knipe H, Nouh M, Skalina T, Redating pregnancy (ultrasound). Reference article, Radiopaedia.org (Accessed on 27 Mar 2025) https://doi.org/10.53347/rID-69154
Disclosures:
At the time the article was last revised Henry Knipe had the following disclosures:
- Integral Diagnostics, Shareholder (ongoing)
- Micro-X Ltd, Shareholder (ongoing)
These were assessed during peer review and were determined to
not be relevant to the changes that were made.
View Henry Knipe's current disclosures
Redating a pregnancy may occur when there is a discrepancy between the estimated due date (EDD) calculated by the last menstrual period (LMP) and that calculated by ultrasound. Care should be taken when redating a pregnancy, especially in the third trimester, as there may be other reasons for a fetus to be small for gestational age.
According to American College of Obstetricians and Gynaecologists (ACOG), redating is more reliable using the earliest available and reliable ultrasound examination surrogates. For instance, CRL in the first trimester would be more accurate than uncertain LMP or second trimester biometrics.
The estimated timelines listed below mandates the redating of pregnancy when these differences happen on obstetric US exams.
2017 guidelines from The American College of Obstetricians and Gynaecologists (ACOG) recommend consideration of redating when gestational age via LMP is discrepant with gestational age via ultrasound as below 1:
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first trimester (measured via CRL)
≤8+6 weeks: >5 days
9 to 13+6 weeks: >7 days
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second and third trimesters (measured via BPD, HC, AC, FL)
14 to 15+6 weeks: >7 days
16 to 21+6 weeks: >10 days
22 to 27+6 weeks: >14 days
>28 weeks: >21 days
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1. Methods for estimating the due date. Committee Opinion No. 700. American College of Obstetricians and Gynecologists. Obstet Gynecol 2017;129:e150–4. From: https://www.acog.org/Clinical-Guidance-and-Publications/Committee-Opinions/Committee-on-Obstetric-Practice/Methods-for-Estimating-the-Due-Date
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