Redating pregnancy (ultrasound)

Last revised by Henry Knipe on 2 May 2023

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 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 (e.g. intrauterine growth restriction).

According to American College of Obstetricians and Gynecologists (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 a debatable uncertain LMP or 2nd-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 Gynecologists (ACOG) recommend consideration of redating when gestational age via LMP is discrepant with gestational age via ultrasound as below 1:

  • first trimester (measured via CRL)

    • ≤8+6 weeks: >5 days

    • 9 to 13+6 weeks: >7 days

  • 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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