Trends in “Prematurity” United States: 1950-67
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Analytical Studies Series 3 Number 15 Trendsin “Prematurity” United States: 1950-67 An analysis of the trend in live births registered in the United States, 1950-67 by weight at birth and period of gestation, in cluding an assessment of the quality of the data. Other factors, such as age of mother, plurality, sex, and delivery in hospi~ls are included insofar as they have a bearing on the trends. DHEW Publication No. (HSM) 72-1030 U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE Public Health Service Health Services and Mental Health Administration National Center for Health Statistics Rockvill e, Md. Jonuary 1972 Vital and Health Statistics-Series 3-No. 15 For sale by the Superintendent of Docummts, U.S. Government Prhrtfng Office, Wsshfngton, D.C. 20402- Prioe 55 cents NATIONAL CENTER FOR HEALTH STATISTICS THEODORE D. WOOLSEY, Director PHILIP S. LAWRENCE, SC.D., Associate Director OSWALD K. SAGEN, Ph.D., Assistant Directorfor Health Statistics Development WALT R, SIMMONS, M.A,, Assistant Director for Research and Scientific Development JAMES E. KELLY, D.D.S., Dental Advisor EDWARD E. MINTY, Executive Officer ALICE HAYWOOD, Information Officer OFFICEOF HEALTHSTATISTICS ANALYSIS IWAO M. MORIYAMA, Ph. D., Director DEAN E. KRUEGER, M.S., Deputy Director Vital and Health Statistics-Series 3-No. 15 DHEW Publication No. (HSM) 72-1030 Library of Congress Catalog Card Number 70-610274 CONTENTS Page Introduction. ...1 Definitions . ...3 SourceofData . ...4 CompletenessofData. ...5 AccuracyofData . ...5 Findings . ...8 PeriodofGestation . ...8 Trends . ...8 Color, . .. o . ..10 Future Prospects . .. ...11 WeightatBirth .’....... ...11 Trends . ...11 FutureProspecti . ...13 WeightatBirthandPeriodofGestation . 14 AreasReportingLMP . ...14 Areas ReportingWeeks of Gestation . 16 AgeofMother . ...17 Birth Order . ...19 Plurality . ...19 Sex . ...20 P1aceofBirthandAttendant. ...20 InternationalCompariso n... ...26 SummaryandConclusion . ...27 Summary . ...27 Quality of Data . ...28 Conclusion . ...29 References . ...30 ListofDetailedTables . ...33 TechnicalAppendix . ...47 DefinitionofLiveBirth . ...47 SourcesofData . ...47 NatalityStatistics . ...47 StandardCertificateofLiveBirth . 47 ClassificationofData . ...49 AgeofMother . ...49 TotalBirthOrder . ...49 CONTENTS–Con. Page Race and Color . ..49 Place of Birth . 49 BirthWeight . ...49 PeriodofGestation . ...49 QualityofData . .50 CompletenessofRegistration. ...50 Quality Control Procedures . ...50 Sampling of Birth Records . ..51 SYMBOLS Datanot available---------------------------------------- --- Category not applicable ------------------------------- . Quantity zero ---------------------------------------------- - Quantity more than Obutless than 0.05----- 0.0 Figure does not meet standards of reliability or precision ------------------------------ * iv TRENDS IN “PREMATURITY” UNITED STATES: 1950-67 Helen C. Chase, Dr. P.H., National Academy of Sciences, and Mary E. Byrnes, M.A., Distn”ct of Columbia Department of Human Resourcesa INTRODUCTION In the United States infant mortality declined rapidly and at a fairly constant rate of decline for about the first half of the 20th century (figure 1). Around midcentury, the rate of decline decelerated and appeared to level off for about a decade.l Data for 1966-68 suggest a reassertion of a more rapid decline, but not at the pace experienced prior to 1950. When the data for the United States were compared with those for a group of economi cally and medically advanced Western European countries, it was found that the trends for some of the other countries seemed to be leveling off as well, but at considerably lower Ievels.z$3 For example, as recently as 1966, the infant mor tality rate for the United States (23.7 per 1,000 live births) was higher than the rate for Sweden (12.6), the Netherlands (14.7), and Australia (18.2 ).3 It also appeared that the proportion of infants weighing 2,500 grams (5% pounds) or less at birth (low birth weight infants) was higher among infants born alive in the United 10 ~ 1915 1920 1930 States than in Denmark, the Netherlands, 1940 197C Sweden, or New Zealand.3 Y4 Nationally, the YEAR percent of low birth weight among infants was reported to have been higher in 1960 than in January-March 1950, with practically no change Figure 1. Infant mortality rates: United Statas, 1915-19 to 1967 for white infants but a marked increase for other infants.5 This observation, which was limited to a 3-month period in 1950 and to the year 1960, led to the present study, which covers the 18-year period 1950-67. Among the many factors affecting infant a Dr. Chase is Staff Associate (Blostatistics) at the Health survival, physical underdevelopment and imma Services Study, Institute of Medicine, National Academy of turity at time of birth present the greatest Sciences. Miss Bymes is Health Statistician, Community Health hazards. In the United States, the risk of death Services Adrnhdsttation, Dktrict of Columbia Department of in the first year of life among infants who weigh Human Resources. At the time this study was completecL both authors were statisticians with the National Center for Health 2,500 grams or less at birth is 17 times the risk Statistics. among infants weighing 2,501 grams or more. 1 Studies have shown that this ‘relative risk far urban hospitals z Although an increasing trend exceeds the risk associated with disadvantageous was observed in this institution, it need not birth order, sex, age of mother, or socio necessarily imply a nationwide trend. It may economic leveL6 Two national studies have perhaps reflec{ a change in the composition of shown that the risk differential associated with the hospital’s clientele or in the metropolitan color (a ratio of 2:1, Negro and other minority population. Since the close of World War II, races versus white) is considerably less than that there have been large in-migrations of Negroes associated with low birth weight (17: 1).5 ~7 and other minority groups into large metro Not only are infants of low birth weight politan centers, and at the same time there have subject to a greater risk of death, but they are been out-migrations of the more affluent white also subject to higher morbidity, particularly of population to the suburbs. Because the propor the central nervous system.T’4s A greater preva tion of low birth weight infants is higher among lence of cerebral palsy, epilepsy, mental retarda newborn infants of minority races, the rate in a tion, congenital anomalies, deafness, blindness, single hospital or city is affected by the propor and strabismus has been found among infants of tions of the two subgroups in the population low birth weight and/or curtailed gestation than studied. While the net effect of population among fully developed full-term infants. None changes over time may result in an increase in of these conditions is considered transient in the proportion of low birth weight infants nature; many of them are severe, and often they among those born in a specific hospital or city, are chronic, as well as handicapping conditions. the nationwide proportions must be considered The data on associations between low birth separately. weight and curtailed gestation on the one hand The present study explores the national data and,. excess mortality and morbidity on the other for the period 1950-67 and includes a critical requn-e further” clarification. For some of the review of the changes in the reporting of birth conditions mentioned, low birth weight may be weight and gestation information. Trends by a consequence of the associated condition rather birth weight and gestational age are examined. than a correlate of the causative mechanism. For The two variables are considered simultaneously example, an infant with severe congenital for years for which published data are available. anomalies of the musculoskeletal system may be The proportion of low birth weight infants of low birth weight because the fetus was unable increased slightly from 7.5 percent in 1950 to to grow to a fully developed infant after it was 8.2 percent in 1967. This was because the affected in utero.38 Such an infant may, never proportion among “other” infants increased theless, be full term. On the other hand, other markedly (from 10.2 percent to 13.6) while that conditions may be resultants of low birth weight among white infants remained relatively stable and/or curtailed gestation in the etiologic sense. (around 7 percent). Using gestation data of less In either case, birth weight and the duration of than satisfactory quality, a tentative conclusion gestation together are assuming increasing im was reached that there is no evidence of changes portance, in evaluating the prognosis of newborn in gestation parallel to the changes in distribu infants. tions of birth weight for “other” infants. Other In view of the serious nature of the disorders factors such as age of mother, plurality, sex, and associated with low birth weight, public health is delivery in hospitals did not appear to explain directing increasing attention to the causes and the trends which were observed. consequences of impaired fetal development. Infants of low birth weight or those with The evidence has been derived from a variety of curtailed periods of gestation are subject to studies, some retrospective, some prospective. If much higher mortality and morbidity than their prematurity among newborn infants is truly heavier counterparts, and their number is not increasing, then greater numbers of infants with