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When LMP and Ultrasound Dates Don’t Match: When to Redate?


Historically, dating pregnancies and calculating due dates were left to weekly pregnancy calendars.  However, ultrasound dating, in particular first trimester sonography, has greatly improved our ability to calculate the estimated due date (EDD).  There will be times that dating based on LMP does not match the ultrasound date.

ACOG recommends redating as follows:

  • First trimester: based on CRL measurement
    • 8w6d or less: redate if discrepancy is > 5d
    • 9w0d – 13w6d: redate if discrepancy is > 7d
  • Second trimester: based on BPD, HC, AC and FL
    • 14w0d – 15w6d: redate if discrepancy is > 7d
    • 16w0d – 21w6d: redate if discrepancy is > 10d
    • 22w0d – 27w6d: redate if discrepancy is > 14d
  • Third trimester: based on BPD, HC, AC and FL
    • 28w0d and beyond: redate if discrepancy is > 21d
      • Use caution when redating in the 3rd trimester as discrepancy may reflect growth restriction
      • Management should not be based on ultrasound alone but rather comprehensive clinical assessment


Clinical determination of EDD, 280 days after the last menstrual period (LMP) still plays a role but may not always be accurate due to variability in length of an individual woman’s cycle length or timing of ovulation.  Accurate dating is vital to pregnancy management, as certain interventions and management decisions may be based on such information including timing of delivery in the case of pregnancy complications.


  • First trimester ultrasound is the most accurate time frame for pregnancy dating and can increase the accuracy of the EDD even if LMP is known
  • Consider a pregnancy without a dating ultrasound prior to 22 0/7 weeks ‘suboptimally dated’ (refer to Related ObG Topics below)
  • Mean sac diameter is not recommended for dating
  • In the setting of assisted reproductive technology (ART), the ART derived gestational age should be used for EDD using the age of the embryo and the transfer date
    • The age of the embryo is subtracted from the number of days between ovulation to delivery (280-14 = 266).  For example, if the embryo is 3 days at transfer, the due date is 263 days from the date of transfer.
  • If the CRL is greater than 84 mm, biometric parameters should be used to date the pregnancy
  • Once the EDD has been established using the LMP and/or first accurate ultrasound measurement, it should be recorded in the medical record and discussed with the patient

Learn More – Primary Sources:

ACOG/AIUM/SMFM Committee Opinion 700: Methods for Estimating Due Date

AIUM Practice Parameter for the Performance of Limited Obstetric Ultrasound Examinations by Advanced Clinical Providers

ACOG AIUM Practice Bulletin 175: Ultrasound in Pregnancy