CMECNE

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

Learning Objectives and CME/Disclosure Information

This activity is intended for healthcare providers delivering care to women and their families.

After completing this activity, the participant should be better able to:

1. Name the time in gestation when ultrasound is most accurate
2. Discuss the ACOG recommendations for redating a pregnancy based on trimester

Estimated time to complete activity: 0.25 hours

Faculty:

Susan J. Gross, MD, FRCSC, FACOG, FACMG
President and CEO, The ObG Project

Disclosure of Conflicts of Interest

Postgraduate Institute for Medicine (PIM) requires faculty, planners, and others in control of educational content to disclose all their financial relationships with ineligible companies. All identified conflicts of interest (COI) are thoroughly vetted and mitigated according to PIM policy. PIM is committed to providing its learners with high quality accredited continuing education activities and related materials that promote improvements or quality in healthcare and not a specific proprietary business interest of an ineligible company.

The PIM planners and others have nothing to disclose. The OBG Project planners and others have nothing to disclose.

Faculty: Susan J. Gross, MD, receives consulting fees from Cradle Genomics, and has financial interest in The ObG Project, Inc.

Planners and Managers: The PIM planners and managers, Trace Hutchison, PharmD, Samantha Mattiucci, PharmD, CHCP, Judi Smelker-Mitchek, MBA, MSN, RN, and Jan Schultz, MSN, RN, CHCP have nothing to disclose.

Method of Participation and Request for Credit

Fees for participating and receiving CME credit for this activity are as posted on The ObG Project website. During the period from Jan 25 2022 through Jan 25 2024, participants must read the learning objectives and faculty disclosures and study the educational activity.

If you wish to receive acknowledgment for completing this activity, please complete the test and evaluation. Upon registering and successfully completing the test with a score of 100% and the activity evaluation, your certificate will be made available immediately.

Joint Accreditation Statement

In support of improving patient care, this activity has been planned and implemented by the Postgraduate Institute for Medicine and The ObG Project. Postgraduate Institute for Medicine is jointly accredited by the Accreditation Council for Continuing Medical Education (ACCME), the Accreditation Council for Pharmacy Education (ACPE), and the American Nurses Credentialing Center (ANCC), to provide continuing education for the healthcare team.

Physician Continuing Medical Education

Postgraduate Institute for Medicine designates this enduring material for a maximum of 0.25 AMA PRA Category 1 Credit(s)™. Physicians should claim only the credit commensurate with the extent of their participation in the activity.

Continuing Nursing Education

The maximum number of hours awarded for this Continuing Nursing Education activity is 0.2 contact hours.

Read Disclaimer & Fine Print

CLINICAL ACTIONS:

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

SYNOPSIS:

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.

KEY POINTS:

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

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