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Determination of Gestational Age by Ultrasound
LETTER TO THE EDITOR
Determination of Gestational Age by Ultrasound In Response To the Editor: We read the concerns of Drs. Cooper and Somerset with interest. Before the Guideline1 was published, there was significant discussion among the authors regarding the gestational age cutoffs for which crown-rump length (CRL) should be used to determine gestational age. We agreed that a CRL of 10 mm should be used as the lower limit for the reasons stated by Drs. Cooper and Somerset. We felt that using weeks of gestation would be helpful for obstetrical care providers who did not have ultrasound expertise. It was also noted that in some regions ultrasound reports do not provide the measured CRL, only the equivalent gestational age. This led us to develop guidelines that would be flexible enough to accommodate these situations. The recommendation to which Drs. Cooper and Somerset allude was written as either 7 weeks or 10 mm. Depending on the CRL charts used, there may of course be a discrepancy. The main purpose of the Guideline was to encourage a common method for dating pregnancies for clinical purposes so that the same estimated date of delivery can be easily calculated. We hoped that this would result in less confusion, discordance, and variance among all involved in patient care. The cornerstone of that process was to use early ultrasound information to set the estimated date of delivery in the absence of timed fertilization. We drafted guiding principles with the expectation that individual regions might need flexibility to accommodate local practices and preferences. In the Guideline, we referenced 13 studies that evaluated CRL for estimation of gestational age.1 Each imaging unit, ultrasound machine, and electronic reporting system used in Canada may use different growth curves for reference, and it was not felt to be appropriate to refute or promote the use of a particular growth curve, either for
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CRL or for the other commonly used biometric parameters. In many of the commonly used growth curves, 7þ0 weeks’ gestation is equivalent to a 10 mm CRL.2,3 There would be considerable difficulty in adapting a single chart for use in every unit in the country. Until that happens, we felt it prudent to allow flexibility in the use of growth charts to allow for local preferences and patterns of practice. We want to emphasize how important the quality and precision of the ultrasound is in the interpretation of images. It is entirely reasonable in an individual unit to use the 10 mm cutoff (interpreted with the unit’s preferred growth chart) rather than 7þ0 weeks; we do not feel that this is contradictory, but rather it is in keeping with the intent of the Guideline’s recommendations. We encourage all units in each province to engage and collaborate with others to adopt a provincial standard. We encourage feedback on these comments, and we thank Drs. Cooper and Somerset for their letter. Kimberly Butt, MD, FRCSC Department of Obstetrics and Gynaecology Dr Everett Chalmers Hospital Fredericton NB Ken Lim, MD, FRCSC Department of Obstetrics and Gynaecology University of British Columbia Vancouver BC
REFERENCES 1. Butt K, Lim K, SOGC Diagnostic Imaging Committee. Determination of gestational age by ultrasound. SOGC Clinical Practice Guideline No. 303, February 2014. J Obstet Gynaecol Can 2014;36:171e81. 2. Robinson HP. Gestational age determination: first trimester. In: Chervenak FA, Isaacson GC, Campbell S, editors. Ultrasound in obstetrics and gynecology. Boston: Little Brown; 1993. p. 295e304. 3. Robinson HP, Fleming JE. A critical evaluation of sonar “crown-rump length” measurements. Br J Obstet Gynecol 1975;82:702e10.
J Obstet Gynaecol Can 2016;38(5):432 Copyright ª 2016 The Society of Obstetricians and Gynaecologists of Canada/La Société des obstétriciens et gynécologues du Canada. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jogc.2016.04.003
Determination of Gestational Age by Ultrasound In Response To the Editor: We read the concerns of Drs. Cooper and Somerset with interest. Before the Guideline1 was published, there was significant discussion among the authors regarding the gestational age cutoffs for which crown-rump length (CRL) should be used to determine gestational age. We agreed that a CRL of 10 mm should be used as the lower limit for the reasons stated by Drs. Cooper and Somerset. We felt that using weeks of gestation would be helpful for obstetrical care providers who did not have ultrasound expertise. It was also noted that in some regions ultrasound reports do not provide the measured CRL, only the equivalent gestational age. This led us to develop guidelines that would be flexible enough to accommodate these situations. The recommendation to which Drs. Cooper and Somerset allude was written as either 7 weeks or 10 mm. Depending on the CRL charts used, there may of course be a discrepancy. The main purpose of the Guideline was to encourage a common method for dating pregnancies for clinical purposes so that the same estimated date of delivery can be easily calculated. We hoped that this would result in less confusion, discordance, and variance among all involved in patient care. The cornerstone of that process was to use early ultrasound information to set the estimated date of delivery in the absence of timed fertilization. We drafted guiding principles with the expectation that individual regions might need flexibility to accommodate local practices and preferences. In the Guideline, we referenced 13 studies that evaluated CRL for estimation of gestational age.1 Each imaging unit, ultrasound machine, and electronic reporting system used in Canada may use different growth curves for reference, and it was not felt to be appropriate to refute or promote the use of a particular growth curve, either for
432
l
MAY JOGC MAI 2016
CRL or for the other commonly used biometric parameters. In many of the commonly used growth curves, 7þ0 weeks’ gestation is equivalent to a 10 mm CRL.2,3 There would be considerable difficulty in adapting a single chart for use in every unit in the country. Until that happens, we felt it prudent to allow flexibility in the use of growth charts to allow for local preferences and patterns of practice. We want to emphasize how important the quality and precision of the ultrasound is in the interpretation of images. It is entirely reasonable in an individual unit to use the 10 mm cutoff (interpreted with the unit’s preferred growth chart) rather than 7þ0 weeks; we do not feel that this is contradictory, but rather it is in keeping with the intent of the Guideline’s recommendations. We encourage all units in each province to engage and collaborate with others to adopt a provincial standard. We encourage feedback on these comments, and we thank Drs. Cooper and Somerset for their letter. Kimberly Butt, MD, FRCSC Department of Obstetrics and Gynaecology Dr Everett Chalmers Hospital Fredericton NB Ken Lim, MD, FRCSC Department of Obstetrics and Gynaecology University of British Columbia Vancouver BC
REFERENCES 1. Butt K, Lim K, SOGC Diagnostic Imaging Committee. Determination of gestational age by ultrasound. SOGC Clinical Practice Guideline No. 303, February 2014. J Obstet Gynaecol Can 2014;36:171e81. 2. Robinson HP. Gestational age determination: first trimester. In: Chervenak FA, Isaacson GC, Campbell S, editors. Ultrasound in obstetrics and gynecology. Boston: Little Brown; 1993. p. 295e304. 3. Robinson HP, Fleming JE. A critical evaluation of sonar “crown-rump length” measurements. Br J Obstet Gynecol 1975;82:702e10.
J Obstet Gynaecol Can 2016;38(5):432 Copyright ª 2016 The Society of Obstetricians and Gynaecologists of Canada/La Société des obstétriciens et gynécologues du Canada. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jogc.2016.04.003