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Assigning EDC

Contributor: MFM Division
Last Update: May 2020

Introduction:

Accurate dating of pregnancy is essential to pregnancy care and optimal outcomes. The following steps should be undertaken to establish an EDC:

Step 1: Determine if the LMP is accurate:

By convention, the EDD is 280 days after the first day of the LMP, assuming a regular 28-day menstrual cycle.  Therefore, use of the LMP to establish an EDD requires identification of the first day of the LMP and a regular inter-menstrual interval.

  1. Determine the 1st day of the LMP.
  2. Determine the inter-menstrual.
  3. Assign an LMP-based EDD, adjusting appropriately for the inter-menstrual interval.
  4. If the patient is unsure of her LMP or has an irregular inter-menstrual interval, dating should be based on ultrasound examination.

If pregnancy resulted from IVF, the ART-derived gestational age should be used to assign the EDD as follows:

  1. Determine the age of the embryo.
  2. Determine the embryo transfer date.
  3. EDD = Embryo Transfer Date + 266 – Embryo Age

Step 2: Obtain an ultrasound

First trimester (up to and including 13 6/7 weeks of gestation) ultrasound measurement of crown-rump length of the embryo or fetus is the most accurate method to establish or confirm gestational age.  The CRL measurement should be obtained as follows:

  1. Should be the mean of three discrete measurements.
  2. Should be obtained in a true midsagittal plane, with the genital tubercle and fetal spine longitudinally in view and the maximum length from the cranium to the caudal rump measured as a straight line.
  3. If the CRL exceeds 83 mm, other second-trimester biometric parameters should be used for dating.

Second trimester (14 0/7 – 27 6/7 weeks) and third trimester (28 0/7 weeks and beyond) ultrasound can also be used to assess gestational age, but greater variability exists as compared to first trimester evaluation (Table 1).

Step 3: Select the best clinical estimate for the EDD

  1. Use Table 1 (below) to compare the LMP-based EDD (if applicable) to the ultrasound-based EDD.
  2. The final EDD should be changed to correspond to the ultrasonography dating only if the discrepancy is outside of the gestational age specific range.
  3. Redating in the third trimester, should be done in consultation with an attending physician, to minimize the risks of overlooking a small fetus that may be growth restricted.

Graphic

Step 4: Discuss and document the final EDD

  1. Discuss the method for EDD assignment with the patient.
  2. Clearly document the EDD assignment in the medical record.
  3. Subsequent changes to the EDD should be reserved for rare circumstances, discussed with the patient, and documented clearly in the medical record.

Management of Suboptimal Dating

Pregnancies without an ultrasonographic examination confirming or revising the EDD before 22 0/7 weeks should be considered suboptimally dated. The timing of indicated delivery in a woman with a sub-optimally dated pregnancy should be based on the best clinical estimate of gestational age.

  • No elective delivery should be performed in a woman with a sub-optimally dated pregnancy.
  • Although guidelines for indicated late–preterm and early–term deliveries depend on accurate dating criteria, women with sub-optimally dated pregnancies should be managed according to the same guidelines because of the lack of a superior alternative.
  • For women planning a repeat cesarean section, delivery timing is advised at 39 weeks by the best clinical estimate of gestational age.
  • The best clinical estimate of gestational age should serve as the basis for decisions regarding antenatal corticosteroid use.

Given the lack of reliability for predicting newborn pulmonary outcomes and an inability to predict non-respiratory outcomes, amniocentesis for fetal lung maturity is not recommended as a routine component of decision making when considering delivery in a woman with a sub-optimally dated pregnancy.

An interval ultrasound assessment of fetal weight and gestational age 3-4 weeks after the initial ultrasonographic study should be considered, since it may detect cases of fetal growth restriction. In cases were fetal growth action is suspected, follow-up Doppler studies should be obtained and the pregnancy managed accordingly.

Given the concern that a full-term or late-term sub-optimally dated pregnancy could actually be weeks farther along than expected, initiation of antenatal testing at 39-40 weeks of gestation may be considered. Late-term delivery is indicated at 41 weeks of gestation when gestational age is uncertain, using the best clinical estimate gestational age.

References

  1. Methods for Estimating the Due Date. Committee Opinion No. 700. American College of Obstetricians and Gynecologists. May 2017
  2. Management of Suboptimally Dated Pregnancies. Committee Opinion No. 688. American College of Obstetricians and Gynecologists. March 2017.

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