- Email: [email protected]
Determination of Gestational Age by Ultrasound
LETTER TO THE EDITOR
Determination of Gestational Age by Ultrasound To the Editor: We write to ask for a review and revision of SOGC Clinical Guideline 303, “Determination of Gestational Age by Ultrasound,” published in February 2014.1 The authors of this Guideline reasonably determined that the minimum crown-rump length (CRL) to determine gestational age accurately should be 10 mm. However, in the recommendations they state, “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.” Thus a gestational age (7 weeks) was introduced with a measurement (10 mm). Using the most accurate dating chart currently available,2 a CRL of 10.0 mm is equivalent to a gestational age of 7þ2 weeks, and a CRL of 8.1 mm is equivalent to 7þ0 weeks. In our practice (including primary care and tertiary referrals), we have noted increasing variation the closer to 7þ0 weeks the initial dating scan is performed compared with the 11- to 14-week nuchal scan. Our conclusion has been that the CRL is frequently under-measured in very early dating scans. This is understandable given that the
orientation of the fetus cannot be accurately determined in most cases. Unless the fetus is measured in a true sagittal plane and appropriately magnified to enable accurate caliper placement, anything else may be an underestimate. Any use of CRL below 10 mm for purposes of dating will only exacerbate this error. We would therefore ask that this guideline be revised and any reference to 7þ0 weeks’ gestation for dating be removed, leaving 10.0 mm as the lowest CRL to use for accurate dating. Stephanie Cooper, FRCSC David Somerset, FRCSC, FRCOG, FRANZCOG Southern Alberta Centre for Maternal Fetal Medicine Calgary AB
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. Papaioannou GI, Syngelaki A, Poon LC, Ross JA, Nicolaides KH. Normal ranges of embryonic length, embryonic heart rate, gestational sac diameter and yolk sac diameter at 6-10 weeks. Fetal Diagn Ther 2010;28:207e19.
J Obstet Gynaecol Can 2016;38(4):337 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.2016.03.007
APRIL JOGC AVRIL 2016
l
337
Determination of Gestational Age by Ultrasound To the Editor: We write to ask for a review and revision of SOGC Clinical Guideline 303, “Determination of Gestational Age by Ultrasound,” published in February 2014.1 The authors of this Guideline reasonably determined that the minimum crown-rump length (CRL) to determine gestational age accurately should be 10 mm. However, in the recommendations they state, “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.” Thus a gestational age (7 weeks) was introduced with a measurement (10 mm). Using the most accurate dating chart currently available,2 a CRL of 10.0 mm is equivalent to a gestational age of 7þ2 weeks, and a CRL of 8.1 mm is equivalent to 7þ0 weeks. In our practice (including primary care and tertiary referrals), we have noted increasing variation the closer to 7þ0 weeks the initial dating scan is performed compared with the 11- to 14-week nuchal scan. Our conclusion has been that the CRL is frequently under-measured in very early dating scans. This is understandable given that the
orientation of the fetus cannot be accurately determined in most cases. Unless the fetus is measured in a true sagittal plane and appropriately magnified to enable accurate caliper placement, anything else may be an underestimate. Any use of CRL below 10 mm for purposes of dating will only exacerbate this error. We would therefore ask that this guideline be revised and any reference to 7þ0 weeks’ gestation for dating be removed, leaving 10.0 mm as the lowest CRL to use for accurate dating. Stephanie Cooper, FRCSC David Somerset, FRCSC, FRCOG, FRANZCOG Southern Alberta Centre for Maternal Fetal Medicine Calgary AB
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. Papaioannou GI, Syngelaki A, Poon LC, Ross JA, Nicolaides KH. Normal ranges of embryonic length, embryonic heart rate, gestational sac diameter and yolk sac diameter at 6-10 weeks. Fetal Diagn Ther 2010;28:207e19.
J Obstet Gynaecol Can 2016;38(4):337 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.2016.03.007
APRIL JOGC AVRIL 2016
l
337