Foreword

On 15–16 March 2005 the Division of Vital Statistics, National Center for Health Statistics (NCHS), and the Division of Reproductive Health, National Center for Chronic Disease Prevention and Health Promotion, both of the United States Centers for Disease Control and Prevention (CDC) convened a workshop on the measurement of gestational age and the challenges to conducting surveillance and research using gestational age estimates from US vital statistics. Certificates of livebirth submitted to States' vital statistics registries are still the main source of US population-based information used for perinatal surveillance and research, to describe trends, determine groups at risk, and inform policy and practice relating to maternal and perinatal mortality and morbidity. Because of the importance of gestational age, it is to our benefit to understand the history and limitations of this measure in all of its manifestations. This supplemental issue to Paediatric and Perinatal Epidemiology is the result of the workshop and presents thoughtful commentary, looking back on topics relating to the history of gestational age estimates in vital statistics; on problems with data cleaning; on analyses pinpointing sources of misclassification and bias; on comparisons of estimates among vital statistics and sources that may be expected to be more accurate (California's Expanded Alpha-fetoprotein Screening Program, national assisted reproductive technology surveillance, Swedish vital records). It also looks forward on possible ways out (multiple imputation). Prominent in this issue is a discussion of the ‘bimodal’ distribution of birthweight seen at early gestational ages. Not only are there problems with missing and invalid information for the date of the mother's last normal menstrual period (LMP), which serves as the main measurement of gestational age in vital statistics data, but there also appears to be extensive misclassification of gestational ages at ≤35 weeks based on menstrual dates. This is demonstrated by the proportion of births that are very preterm (<32 weeks) or moderately preterm according to their LMP-based gestational ages, but have birthweights well in excess of 2800 g (‘second birthweight mode’). The disproportionately high birthweights are seen even as early as 28–31 weeks' gestation, which is biologically implausible, and they are presumed to be misclassified term births. Further, the misclassification is not random; high-risk women are much more likely to be represented among those whose neonates are misclassified. It follows that the reported US preterm birth rates to date have been overestimated, and for some high-risk groups more than others, although the precise extent of the overestimation is not fully known because of idiosyncrasies in reporting and our inability to monitor or estimate inaccuracies of recording. On the other hand, because of the errors in the LMP-based gestational age measure, the US rates of gestational age-specific perinatal mortality and morbidity using vital statistics files at gestational ages ≤35 weeks are probably underestimated and give misleading information to clinicians about the probability of survival and morbidity at these early gestational ages. Several recent publications1,2 and papers in this supplement discuss using the ‘clinical estimate’ from the birth certificate to circumvent this problem, but as will be seen from the papers in this issue comparing gestational age estimates among various sources, while this helps, in no analysis was the ‘second mode’ totally eliminated. Inaccuracies in recording and reporting will always be with us, although hopefully minimised. Now, before we have even solved the problems of the past, we are entering a new era of the ‘obstetric estimate’ on the revised birth certificates that is defined specifically as based on ultrasound dating and being phased in nationally (replacing the ‘clinical estimate’). Already, some proportion, and probably a large one in more recent years, of gestational ages using the ‘clinical estimate’ were based on ultrasound. Findings from ultrasonogaphy are being used clinically to ‘modify’ or ‘correct’ the LMP recorded on the prenatal record, and reported time trends in preterm birth may have been influenced by this change in standard practice in ways that are difficult to predict. While the change to the obstetric estimate on the birth certificate may be an improvement, it presents an entirely new set of challenges, especially for surveillance and achieving concordance among methods of gestational age measurement within the national files. For policy and to guide care, though, it is worth our efforts to keep tackling these issues, despite their difficulty and seeming insuperableness. On that note, we would like to acknowledge the untimely passing of one of our best colleagues and fellow tackle on the front line, Greg R. Alexander, MPH, ScD, who made enormous contributions to the field of maternal and child health, in general, and to our understanding of the uses and misuses of vital statistics. Juan Manuel Acuña, MD, Division of Reproductive Health, NCCDPHP, CDC, DHHS Marilee C. Allen, MD, Johns Hopkins University School of Medicine, Baltimore, MD Cande V. Ananth, PhD, MPH, UMDNJ-Robert Wood Johnson Medical School, New Brunswick, NJ William M. Callaghan, MD, MPH, Division of Reproductive Health, NCCDPHP, CDC, DHHS Karla Damus, RN, PhD, National March of Dimes, White Plains, NY Patricia M. Dietz, DrPH, MPH, Division of Reproductive Health, NCCDPHP, CDC, DHHS Lucinda J. England, MD, MSPH, Division of Reproductive Health, NCCDPHP, CDC, DHHS Bengt Haglund, PhD, National Board of Health and Welfare, Stockholm, Sweden K.S. Joseph, MD, MPH, Dalhousie University, Halifax, Nova Scotia, Canada John L. Kiely, PhD, Office of Analysis and Epidemiology, NCHS, CDC, DHHS Sharon E. W. Kirmeyer, PhD, Division of Vital Statistics, NCHS, CDC, DHHS Mark A. Klebanoff, MD, MPH, DESPR, NICHD, NIH, DHHS Joyce A. Martin, MPH, Division of Vital Statistics, NCHS, CDC, DHHS Jennifer D. Parker, PhD, Office of Analysis and Epidemiology, NCHS, CDC, DHHS Cheng Qin, MD, DrPH, Division of Reproductive Health, NCCDPHP, CDC, DHHS Kenneth C. Schoendorf, MD, MPH, Office of Analysis and Epidemiology, NCHS, CDC, DHHS Anjel Vahratian, PhD, MPH, University of Michigan School of Medicine, Ann Arbor, MI Jun (Jim) Zhang, MD, PhD, DESPR, NICHD, NIH, DHHS Lara Akinbami, MD, Office of Analysis and Epidemiology, NCHS, CDC, DHHS Amy M. Branum, MSPH, Office of Analysis and Epidemiology, NCHS, CDC, DHHS Carol Bruce, MPH, RN, Division of Reproductive Health, NCCDPHP, CDC, DHHS Cynthia Ferré, PhD, Division of Reproductive Health, NCCDPHP, CDC, DHHS Barry I. Graubard, PhD, Division of Cancer Epidemiology and Genetics, NCI, NIH, DHHS Brady E. Hamilton, PhD, Division of Vital Statistics, NCHS, CDC, DHHS Mary L. Hediger, PhD, DESPR, NICHD, NIH, DHHS Jason Hsia, PhD, Division of Reproductive Health, NCCDPHP, CDC, DHHS Michele Kiely, DrPH, DESPR, NICHD, NIH, DHHS Martin Kharrazi, PhD, California Department of Health Services, Richmond, CA Susan L. Lukacs, DO, MPH, Office of Analysis and Epidemiology, NCHS, CDC, DHHS Marian F. MacDorman, PhD, Division of Vital Statistics, NCHS, CDC, DHHS T. J. Mathews, MS, Division of Vital Statistics, NCHS, CDC, DHHS Fay Menacker, DrPH, RN, Division of Vital Statistics, NCHS, CDC, DHHS Martha L. Munson, MS, Division of Vital Statistics, NCHS, CDC, DHHS Samuel Notzon, PhD, Office of the Center Director, NCHS, CDC, DHHS Samuel F. Posner, PhD, Division of Reproductive Health, NCCDPHP, CDC, DHHS Sonja A. Rasmussen, MD, Division of Reproductive Health, NCCDPHP, CDC, DHHS Charles Rothwell, MS, Director, Division of Vital Statistics, NCHS, CDC, DHHS Michelle Pearl, PhD, Sequoia Foundation, California Department of Health Services, Richmond, CA Nathaniel Schenker, PhD, Office of Research and Methodology, NCHS, CDC, DHHS Paul D. Sutton, PhD, Division of Vital Statistics, NCHS, CDC, DHHS Stephanie J. Ventura, MA, Division of Vital Statistics, NCHS, CDC, DHHS Megan L. Wier, MPH, Sequoia Foundation, California Department of Health Services, Richmond, CA

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