DOI: 10.1002/pd.4238
CORRESPONDENCE
Response to “On gestational weeks and maths”
When making general statements in obstetrics, it is standard practice to express gestational age, as we have done, as the number of completed weeks of gestation.1,2 This is similar to the common parlance used when postnatal age is expressed in completed years: for example, 21-year-olds are those who have passed their 21st but not their 22nd birthday. The usage should not be considered ‘an arithmetic aberration’ but instead a simplification. Our statements about the acceptable range of gestation for biochemical and ultrasound screening were based on model-predicted performance, published according to completed weeks. The model parameters were derived from meta-analysis of large studies, where for each woman, the screening marker levels had been adjusted for gestational age in a more precise way, generally using weeks and days. We agree with Dr Hummel that clinical practice differs regarding how to calculate gestation, both when considering whether an individual woman is within the acceptable range and, if so, in the gestational age-specific adjustment of her markers. It has been demonstrated, at least for second trimester markers, that ultrasound biometry yields a more precise screening result than the time since the first day of the last menstrual period (‘dates’).3 Nevertheless, if both biometry and dates are available, some clinicians only use biometry if the difference between them is large or the dates are uncertain or where cycles are irregular or follow oral contraceptive pill withdrawal. Moreover, there are many published equations in use that convert an ultrasound biometric measurement to gestational age.4 There are indeed further difficulties for pregnancies resulting from assisted reproductive technologies. Changes in marker levels with gestation age could reflect size as well as fetal development so that gestation calculated from ultrasound biometry may be a better basis for marker adjustment than the time since implantation.
Prenatal Diagnosis 2013, 33, 1218–1219
Peter Benn1*, Antoni Borrell2, Rossa Chiu3, Howard Cuckle4, Lorraine Dugoff5, Brigitte Faas6, Susan Gross7, Joann Johnson8, Ron Maymon9, Mary Norton10, Anthony Odibo11, Peter Schielen12, Kevin Spencer13, Tianhua Huang14, Dave Wright15 and Yuval Yaron16 1
Department of Genetics and Developmental Biology, University of Connecticut Health Center, Farmington, CT, USA 2 Prenatal Diagnosis Unit, Institute of Gynecology, Obstetrics and Neonatology, Hospital Clinic, Maternitat Campus, University of Barcelona Medical School, Catalonia, Spain 3 Department of Chemical Pathology, The Chinese University of Hong Kong, Prince of Wales Hospital, Hong Kong 4 Department of Obstetrics and Gynecology, Columbia University Medical Center, New York, NY, USA 5 Department of Obstetrics and Gynecology, University of Pennsylvania, Philadelphia, PA, USA 6 Department of Human Genetics, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands 7 Department of Obstetrics and Gynecology, Albert Einstein College of Medicine, New York, NY, USA 8 Department of Obstetrics and Gynecology, University of Calgary, Calgary, AB, Canada 9 Department of Obstetrics and Gynecology, Assaf Harofe Medical Center, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel 10 Department of Obstetrics and Gynecology, Stanford University School of Medicine, Stanford, CA, USA 11 Department of Obstetrics and Gynecology, Washington University in St Louis, St Louis, MO, USA 12 Laboratory for Infectious Diseases and Perinatal Screening, National Institute for Public Health and the Environment (RIVM), Bilthoven, The Netherlands 13 Prenatal Screening Unit, Clinical Biochemistry Department, Barking Havering & Redbridge University Hospitals, King George Hospital, Goodmayes, UK 14 Genetics Program, North York General Hospital, Toronto, ON, Canada 15 Department of Mathematics and Statistics, University of Plymouth, Plymouth, UK 16 Prenatal Diagnosis Unit Genetic Institute, Sourasky Medical Center, Tel Aviv, Israel * Correspondence to: Peter Benn. E-mail: [email protected] Funding sources: None Conflicts of interest: None declared
© 2013 John Wiley & Sons, Ltd.
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REFERENCES 1. European Commission. Eurostat. Statistics Explained. http://epp. eurostat.ec.europa.eu/statistics_explained/index.php/Glossary: Gestational_age Accessed August 22, 2013. 2. Chabra S. Concept of gestational age in “completed weeks”: lost in translation. Obstet Gynecol. 2012;119:183–84.
Prenatal Diagnosis 2013, 33, 1218–1219
3. Rahim RR, Cuckle HS, Sehmi IK, Jones RG. Ultrasound dating policy in maternal serum screening for Down’s syndrome. Prenat Diagn 2002;22:1181–4. 4. Loughna P, Chitty L, Evans T, Chudleigh T. Fetal size and dating: charts recommended for clinical obstetric practice. Ultrasound 2009;17:161–7.
© 2013 John Wiley & Sons, Ltd.
CORRESPONDENCE
Response to “On gestational weeks and maths”
When making general statements in obstetrics, it is standard practice to express gestational age, as we have done, as the number of completed weeks of gestation.1,2 This is similar to the common parlance used when postnatal age is expressed in completed years: for example, 21-year-olds are those who have passed their 21st but not their 22nd birthday. The usage should not be considered ‘an arithmetic aberration’ but instead a simplification. Our statements about the acceptable range of gestation for biochemical and ultrasound screening were based on model-predicted performance, published according to completed weeks. The model parameters were derived from meta-analysis of large studies, where for each woman, the screening marker levels had been adjusted for gestational age in a more precise way, generally using weeks and days. We agree with Dr Hummel that clinical practice differs regarding how to calculate gestation, both when considering whether an individual woman is within the acceptable range and, if so, in the gestational age-specific adjustment of her markers. It has been demonstrated, at least for second trimester markers, that ultrasound biometry yields a more precise screening result than the time since the first day of the last menstrual period (‘dates’).3 Nevertheless, if both biometry and dates are available, some clinicians only use biometry if the difference between them is large or the dates are uncertain or where cycles are irregular or follow oral contraceptive pill withdrawal. Moreover, there are many published equations in use that convert an ultrasound biometric measurement to gestational age.4 There are indeed further difficulties for pregnancies resulting from assisted reproductive technologies. Changes in marker levels with gestation age could reflect size as well as fetal development so that gestation calculated from ultrasound biometry may be a better basis for marker adjustment than the time since implantation.
Prenatal Diagnosis 2013, 33, 1218–1219
Peter Benn1*, Antoni Borrell2, Rossa Chiu3, Howard Cuckle4, Lorraine Dugoff5, Brigitte Faas6, Susan Gross7, Joann Johnson8, Ron Maymon9, Mary Norton10, Anthony Odibo11, Peter Schielen12, Kevin Spencer13, Tianhua Huang14, Dave Wright15 and Yuval Yaron16 1
Department of Genetics and Developmental Biology, University of Connecticut Health Center, Farmington, CT, USA 2 Prenatal Diagnosis Unit, Institute of Gynecology, Obstetrics and Neonatology, Hospital Clinic, Maternitat Campus, University of Barcelona Medical School, Catalonia, Spain 3 Department of Chemical Pathology, The Chinese University of Hong Kong, Prince of Wales Hospital, Hong Kong 4 Department of Obstetrics and Gynecology, Columbia University Medical Center, New York, NY, USA 5 Department of Obstetrics and Gynecology, University of Pennsylvania, Philadelphia, PA, USA 6 Department of Human Genetics, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands 7 Department of Obstetrics and Gynecology, Albert Einstein College of Medicine, New York, NY, USA 8 Department of Obstetrics and Gynecology, University of Calgary, Calgary, AB, Canada 9 Department of Obstetrics and Gynecology, Assaf Harofe Medical Center, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel 10 Department of Obstetrics and Gynecology, Stanford University School of Medicine, Stanford, CA, USA 11 Department of Obstetrics and Gynecology, Washington University in St Louis, St Louis, MO, USA 12 Laboratory for Infectious Diseases and Perinatal Screening, National Institute for Public Health and the Environment (RIVM), Bilthoven, The Netherlands 13 Prenatal Screening Unit, Clinical Biochemistry Department, Barking Havering & Redbridge University Hospitals, King George Hospital, Goodmayes, UK 14 Genetics Program, North York General Hospital, Toronto, ON, Canada 15 Department of Mathematics and Statistics, University of Plymouth, Plymouth, UK 16 Prenatal Diagnosis Unit Genetic Institute, Sourasky Medical Center, Tel Aviv, Israel * Correspondence to: Peter Benn. E-mail: [email protected] Funding sources: None Conflicts of interest: None declared
© 2013 John Wiley & Sons, Ltd.
1219
REFERENCES 1. European Commission. Eurostat. Statistics Explained. http://epp. eurostat.ec.europa.eu/statistics_explained/index.php/Glossary: Gestational_age Accessed August 22, 2013. 2. Chabra S. Concept of gestational age in “completed weeks”: lost in translation. Obstet Gynecol. 2012;119:183–84.
Prenatal Diagnosis 2013, 33, 1218–1219
3. Rahim RR, Cuckle HS, Sehmi IK, Jones RG. Ultrasound dating policy in maternal serum screening for Down’s syndrome. Prenat Diagn 2002;22:1181–4. 4. Loughna P, Chitty L, Evans T, Chudleigh T. Fetal size and dating: charts recommended for clinical obstetric practice. Ultrasound 2009;17:161–7.
© 2013 John Wiley & Sons, Ltd.
