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
Susan J. Gross, MD, FRCSC, FACOG, FACMG President and CEO, The ObG Project
Disclosure of Conflicts of Interest
Postgraduate Institute for Medicine (PIM) requires instructors, planners, managers and other individuals who are in a position to control the content of this activity to disclose any real or apparent conflict of interest (COI) they may have as related to the content of this activity. All identified COI are thoroughly vetted and resolved according to PIM policy. PIM is committed to providing its learners with high quality CME activities and related materials that promote improvements or quality in healthcare and not a specific proprietary business interest of a commercial interest.
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 Dec 31 2017 through Dec 31 2021, 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 post-test and evaluation. Upon registering and successfully completing the post-test with a score of 100% and the activity evaluation, your certificate will be made available immediately.
For Pharmacists: Upon successfully completing the post-test with a score of 100% and the activity evaluation form, transcript information will be sent to the NABP CPE Monitor Service within 4 weeks.
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.
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
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Disclosure of Unlabeled Use
This educational activity may contain discussion of published and/or investigational uses of agents that are not indicated by the FDA. The planners of this activity do not recommend the use of any agent outside of the labeled indications.
The opinions expressed in the educational activity are those of the faculty and do not necessarily represent the views of the planners. Please refer to the official prescribing information for each product for discussion of approved indications, contraindications, and warnings.
Participants have an implied responsibility to use the newly acquired information to enhance patient outcomes and their own professional development. The information
presented in this activity is not meant to serve as a guideline for patient management. Any procedures, medications, or other courses of diagnosis or treatment discussed or suggested in this activity should not be used by clinicians without evaluation of their patient’s conditions and possible contraindications and/or dangers in use, review of any applicable manufacturer’s product information, and comparison with recommendations of other authorities.
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