New Guidelines Standardize Pregnancy Due Date Estimates

Troy Brown, RN

September 25, 2014

New pregnancy due date estimation guidelines will help correct current variability in the calculation of pregnancy due dates and gestational age and facilitate consistent care of pregnant women and their babies, key medical groups say.

The new guidelines for calculating the estimated due date (EDD) were developed by the American College of Obstetricians and Gynecologists (ACOG), the American Institute of Ultrasound in Medicine (AIUM), and the Society for Maternal-Fetal Medicine (SMFM) and will be published concurrently in Obstetrics & Gynecology and the Journal of Ultrasound in Medicine.

"An accurately assigned EDD is among the most important results of evaluation and history taking early in prenatal care," the authors write. "This information is vital for timing of appropriate obstetric care, scheduling and interpretation of certain antepartum tests, determining the appropriateness of fetal growth, and designing interventions to prevent preterm births, postterm births, and related morbidities.... A consistent and exacting approach to accurate dating is also a research and public health imperative because of the influence of dating on investigational protocols and vital statistics," they add.

The guidelines emphasize the importance of high-quality ultrasound in the first trimester of pregnancy. Calculation of the estimated due date has traditionally been based on the first day of the last menstrual period, but using this method alone can be unreliable because only about half of women can accurately recall their last menstrual period, the authors note. In addition, menstrual cycles may be irregular in length, and the timing of ovulation can vary.

The guidelines also address calculation of the EDD in the second and third trimesters of pregnancy.

"A uniform approach to assigning an EDD will be helpful for clinicians in all types of pregnancies, particularly with women who have had prior medical problems during pregnancy," James D. Goldberg, MD, vice chair of ACOG's Committee on Obstetric Practice, which developed the committee opinion, said in a news release.

"For some women, especially ones with early labor or other complications in the past, like a prior vertical cesarean incision, having an accurate due date is very important for making safe plans for care during their current pregnancy and for timing delivery," Joshua A. Copel, MD, the ACOG's liaison member from AIUM, noted in the release.


  • High-quality ultrasound measurement of the embryo or fetus during the first trimester of pregnancy is the most accurate method of establishing or confirming gestational age.

  • If the pregnancy is the result of assisted reproductive technology (ART), the clinician should use the ART-derived gestational age to assign the EDD. For example, for a pregnancy that results from in vitro fertilization, the clinician should use the age of the embryo and the date of the transfer to establish the EDD.

  • As soon as the clinician has data from the last menstrual period, the first accurate ultrasound examination, or both, the gestational age and the EDD should be calculated, discussed with the patient, and recorded clearly in the patient's medical record.

  • For research and surveillance purposes, the clinician should use the best obstetric estimate, rather than calculations based only on the last menstrual period, to determine gestational age.

  • Subsequent changes to the EDD should only be made in rare circumstances, should be discussed with the patient, and should be recorded clearly in the patient's medical record.

"This is good for our patients. There has been significant variability in the way different obstetrical providers calculated a women's due date; and this standardization will allow for better consistency of care. This new standard may improve outcomes in high risk conditions that require scheduled delivery at term as well as in post-term pregnancies," Sean C. Blackwell, MD, the ACOG's liaison member from SMFM, said in the news release.

"Method for Estimating Due Date." ACOG, AIUM, and SMFM. Full text


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