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OB Guideline 7: Pregnancy Dating/Estimated Date of Delivery


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OB Guideline 7: Pregnancy Dating/Estimated Date of Delivery

Related to: Clinical Guidelines, Communication, Documentation, Informed Consent, Nursing, Obstetrics, Teamwork Training

Accurate assessment of gestational age is of paramount importance for management of pregnancy, interpretation of test results, and timing of interventions. Pregnancy dating should be determined by a combination of the historical, clinical, and laboratory criteria listed below:

  1. known normal last menstrual period, corrected for cycle length;
  2. known date of assisted reproductive technologic intervention;
  3. date(s) of HCG testing (urine or blood);
  4. uterine size on initial physical exam;
  5. date of detection of FHT with Doppler;
  6. first trimester ultrasound measurement of crown-rump length; and
  7. second trimester ultrasound measurement of multiple fetal biometrics.

Ultrasound dating should take precedence over clinical dating if:

  1. the last menstrual period is uncertain or abnormal,
  2. there is a ≥  7 day discrepancy between historical/clinical parameters and first trimester ultrasound measurements, or
  3. there is a ≥ 10 day discrepancy between historical/clinical parameters and second trimester ultrasound measurements (up to 20 weeks gestation).1

Gestational age based on third trimester ultrasound should be interpreted with caution, due to the decreased accuracy of ultrasound for dating late in pregnancy.

When necessary for clinical decision making, fetal pulmonary maturity may be presumed if early reliable dating confirms that the patient is no more than seven days before her estimated date of delivery (EDD).

Alternatively, fetal lung maturity may be assessed by testing the amniotic fluid.However, a mature FLM does not in and of itself guarantee a good neonatal outcome and should not be used as a reason for early delivery without a clinical indication for delivery.

The primary obstetrical provider is responsible for establishing the EDD and documenting this information in the patient’s prenatal record. The sooner the EDD is established and documented in the patient’s prenatal record, the better.


  1. Ultrasonography in pregnancy. ACOG Practice Bulletin No. 101. American College of Obstetricians and Gynecologists. Obstet Gynecol. 2009;113: 451–61.


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May 1, 2014
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