Guideline No. 388-Determination of Gestational Age by Ultrasound



      To assist clinicians in assigning gestational age based on ultrasound biometry.


      To determine whether ultrasound dating provides more accurate gestational age assessment than menstrual dating with or without the use of ultrasound. To provide maternity health care providers and researchers with evidence-based guidelines for the assignment of gestational age. To determine which ultrasound biometric parameters are superior when gestational age is uncertain. To determine whether ultrasound gestational age assessment is cost effective.


      Published literature was retrieved through searches of PubMed or MEDLINE and The Cochrane Library in 2013 using appropriate controlled vocabulary and key words (gestational age, ultrasound biometry, ultrasound dating). Results were restricted to systematic reviews, randomized control trials/controlled clinical trials, and observational studies written in English. There were no date restrictions. Searches were updated on a regular basis and incorporated in the guideline to July 31, 2013. Grey (unpublished) literature was identified through searching the websites of health technology assessment and health technology-related agencies, clinical practice guideline collections, clinical trial registries, and national and international medical specialty societies.


      The quality of evidence in this document was rated using the criteria described in the Report of the Canadian Task Force on Preventive Health Care ( Table 1).

      Benefits, harms, and costs

      Accurate assignment of gestational age may reduce post-dates labour induction and may improve obstetric care through allowing the optimal timing of necessary interventions and the avoidance of unnecessary ones. More accurate dating allows for optimal performance of prenatal screening tests for aneuploidy. A national algorithm for the assignment of gestational age may reduce practice variations across Canada for clinicians and researchers. Potential harms include the possible reassignment of dates when significant fetal pathology (such as fetal growth restriction or macrosomia) result in a discrepancy between ultrasound biometric and clinical gestational age. Such reassignment may lead to the omission of appropriate—or the performance of inappropriate—fetal interventions.

      Summary Statements

      • 1
        When performed with quality and precision, ultrasound alone is more accurate than a “certain” menstrual date for determining gestational age in the first and second trimesters (≤ 23 weeks) in spontaneous conceptions, and it is the best method for estimating the delivery date (II).
      • 2
        In the absence of better assessment of gestational age, routine ultrasound in the first or second trimester reduces inductions for post-term pregnancies (I).
      • 3
        Ideally, every pregnant woman should be offered a first-trimester dating ultrasound; however, if the availability of obstetrical ultrasound is limited, it is reasonable to use a second-trimester scan to assess gestational age (I).


      • 1
        First-trimester crown–rump length is the best parameter for determining gestational age and should be used whenever appropriate (I A).
      • 2
        If there is more than one first-trimester scan with a mean sac diameter or crown–rump length measurement, the earliest ultrasound with a crown–rump length equivalent to at least 7 weeks (or 10 mm) should be used to determine the gestational age (III B). Ideally the dating ultrasound is at least 7 weeks or 10 mm of gestation. However, in the absence of timed fertilization, clinical judgement and discretion can be applied should the only early crown–rump length scan be prior to 10 mm and 7 weeks and thus a repeat scan is not mandatory. Factors to consider include the quality of the scan, ultrasound method, and all available clinical information.
      • 3
        Between the 12th and 14th weeks, crown–rump length and biparietal diameter are similar in accuracy. It is recommended that crown-rump length be used up to 84 mm, and the biparietal diameter be used for measurements > 84 mm (II-1 A).
      • 4
        If a second- or third-trimester scan is used to determine gestational age, a combination of multiple biometric parameters (biparietal diameter, head circumference, abdominal circumference, and femur length) should be used to determine gestational age, rather than a single parameter (II-1 A).
      • 5
        When the assignment of gestational age is based on a third-trimester ultrasound, it is difficult to confirm an accurate due date. Follow-up of interval growth is suggested 2 to 3 weeks following the ultrasound (III C).

      Key Words


      ART ( assisted reproductive technology), BPD ( biparietal diameter), CRL ( crown–rump length), EDD ( estimated due date), LMP ( last menstrual period), MSD ( mean sac diameter)
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