PEDIATRICS Vol. 118 No. 6 December 2006, pp. 2488-2497 (doi:10.1542/10.1542/peds.2006-1824)
ARTICLE |
The Changing Risk of Infant Mortality by Gestation, Plurality, and Race: 1989–1991 Versus 1999–2001
a School of Nursing and Health Studies, University of Miami, Coral Gables, Florida
b Departments of Obstetrics and Gynecology and Pediatrics, Miller School of Medicine, University of Miami, Miami, Florida
c Department of Biostatistics, School of Public Health, University of Michigan, Ann Arbor, Michigan
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ABSTRACT |
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 TOP ABSTRACT METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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OBJECTIVE. Our aim was to quantify contemporary infant mortality risks and to evaluate the change by plurality, gestation, and race during the most recent decade.
PATIENTS AND METHODS. The study population included live births of 20 to 43 weeks' gestation from the 1989–1991 and 1999–2001 US Birth Cohort Linked Birth/Infant Death Data Sets, including 11317895 and 11181095 live births and 89823 and 67129 infant deaths, respectively. Adjusted odds ratios and 95% confidence intervals were calculated to evaluate the change in risk by plurality and gestation and to compare the change with that for singletons.
RESULTS. Overall, the infant mortality risk decreased significantly for singletons, twins, and triplets but nonsignificantly for quadruplets and quintuplets. Compared with singletons, significantly greater reductions were experienced by twins overall and at <37 weeks and triplets at <29 weeks. The largest reduction was for triplets at 20 to 24 weeks and for quadruplets and quintuplets at 25 to 28 weeks. For white infants, significant reductions were achieved overall for singletons, twins, and triplets and at every gestation. For black infants, significant reductions occurred for singletons overall and at every gestation, for twins at <37 weeks, and for triplets at 25 to 28 weeks. Compared with white infants, black infants had significantly lower risks before and higher risks after 33 weeks, although between 1989–1991 and 1999–2001 this survival advantage at earlier ages diminished, and the risk at later gestations increased.
CONCLUSIONS. The improvements in survival were greater for multiples versus singletons and for white versus black infants. Within each plurality, at each gestation the racial disparity in mortality has widened.
Key Words: infant mortality • birth weight • gestation • plurality • racial disparities
Abbreviations: RDS—respiratory distress syndrome • SIDS—sudden infant death syndrome • NCHS—National Center for Health Statistics • AOR—adjusted odds ratio • CI—confidence interval • ART—assisted reproductive technology
During the 1990s, a series of major changes in medical practice, clinical guidelines, and national recommendations occurred in the United States and other industrialized nations, paralleling the rise in multiple pregnancies and potentially affecting national neonatal and postneonatal mortality risks. First, the rapid rise in multiple births, which occurred during this decade, accounted for 2.3% and 3.1% of all live births in 1990 and 2000, respectively; triplets and higher-order multiples accounted for an increasing proportion of multiple births (3.1% and 5.8%, respectively). An estimated 53% of all multiple births, including 80% of triplet and higher-order births, are the result of assisted reproductive technologies,1,2 which have been reported to be at higher risk for adverse pregnancy outcomes.3–6 The use of assisted reproductive technologies carries a 30% to 50% risk of resulting in a multiple pregnancy, depending on the medications and techniques used. Older maternal age requires more aggressive therapies to achieve a pregnancy, including transferring more embryos. Triplet and higher-order pregnancies (quadruplets and quintuplets), which were uncommon before the widespread use of assisted reproductive technologies, are the fastest growing multiple pregnancies, rising more than fourfold since 1980. Born an average of nearly 7 weeks earlier and at half the birth weight of singletons, triplets comprise >90% of higher-order multiples. The 2 most important factors affecting perinatal mortality are gestational age and relative birth weight; with each additional fetus, both of these factors are compromised. Compared with singletons, infants of multiple pregnancies are much more likely to be born early preterm and very low birth weight, important factors contributing to their excess morbidity and mortality.
Second, the threshold of viability was lowered and the survival of extremely low birth-weight infants improved in the 1990s.7 Survival rates during the second half of the last decade were 11% for 401 to 500 g, 27% for 501 to 600 g, 63% for 601 to 700 g, 74% for 701 to 800 g, and 30% for 23 weeks' gestation; 52% for 24 weeks' gestation; and 76% for 25 weeks' gestation.7 Although perinatal mortality has declined from the 1980s through the 1990s, perinatally related disability among survivors has remained unchanged or has risen, depending on the population studied.8–23 For very low birth-weight infants (<1500 g), mortality rates declined by 
50% in the early 1990s,24,25 but since 1995, no additional improvements in mortality or morbidity have been seen, ending a long-term trend of improving outcomes for these infants.25,26
Third, major changes occurred in the contemporary management of premature births, including the widespread use of antenatal corticosteroids and postnatal surfactant.27–29 In 1993, the results of 3 randomized clinical trials of intrathecal instillation of surfactant, administered for the prevention or treatment of neonatal respiratory distress syndrome (RDS), reduced the severity of early respiratory disease and the morbidity and mortality associated with RDS.27,30,31 In 1994, the National Institutes of Health issued consensus statement 95 on the effect of corticosteroids for fetal maturation on perinatal outcomes.32 The consensus panel concluded that antenatal corticosteroid therapy for fetal maturation reduces mortality, RDS, and intraventricular hemorrhage in preterm infants and was a rare example of a technology that yields substantial cost savings in addition to improving health. Subsequent research indicated that with this intervention, the risks for mortality or major disability were comparable for very low birth-weight infants from singleton and multiple pregnancies.33 Although postneonatal corticosteroid therapy demonstrated short-term pulmonary benefits, survival did not improve,34 and associated increases in neurodevelopmental delays and cerebral palsy raised serious concerns about long-term adverse sequelae. For these reasons, the American Academy of Pediatrics 2002 Committee Statement recommended against the routine use of systemic postnatal corticosteroids for the prevention or treatment of chronic lung disease in very low birth-weight infants.35
Fourth is the implementation of national guidelines for infant sleeping positions to reduce sudden infant death syndrome (SIDS) deaths. In 1992, the American Academy of Pediatrics Task Force on Infant Positioning and SIDS issued its first recommendation that infants be put to sleep in a supine or side position.36 The Back to Sleep campaign was initiated in 1994 to inform the public about the risks associated with prone sleep positioning of infants, with the goal of further reducing the prevalence of prone positioning in the United States.37 Between 1992 and 1996, the prevalence of prone sleep positioning fell from 70% to 24%, paralleling a 38% decline in overall SIDS mortality for the United States.38,39 These declines were smaller for black than for nonblack populations, reflecting a higher prevalence of prone positioning in the former group.40 The purpose of this study was to quantify infant mortality risks by gestation, plurality, and race and to evaluate how medical advances and national recommendations have influenced these risks during the 1990s.
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METHODS |
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The data sets for this study included the Birth Cohort Linked Birth/Infant Death Data Set for 1989–1991 and 1999–2001, the earliest and most recent years for which plurality-specific data are available. The methodology takes advantage of 2 existing data sources: state-linked files for the identification of linked birth and infant death certificates, and National Center for Health Statistics (NCHS) natality and mortality computerized statistical files, the source of computer records for the 2 linked certificates. All states link infant death certificates with their corresponding birth certificates for legal and statistical purposes. When the birth and death of an infant occur in different states, copies of the records are exchanged by the state of death and the state of birth to affect a link. In addition, if a third state is identified as the state of residence at the time of birth or death, that state is also sent a copy of the appropriate certificate by the state where the birth or death occurred. The NCHS natality and mortality files, produced annually, include statistical data from birth and death certificates that are provided to NCHS by states under the Vital Statistics Cooperative Program. The data have been coded according to uniform coding specifications, have passed rigid quality control standards, have been edited and reviewed, and are the basis for official US birth and death statistics. To initiate processing, NCHS obtains matching birth certificate numbers from states for all infant deaths that occurred in their jurisdiction. This information is used to extract final, edited mortality and natality data from the NCHS natality and mortality statistical files. Individual birth and death records are selected from their respective files and linked into a single statistical record, thereby establishing a national linked record file. After the initial linkage, NCHS returns to the states where the deaths occurred with computer lists of unlinked infant death certificates for follow-up linking. If the birth occurred in a state different from the state of death, the state of birth identified on the death certificate is contacted to obtain the linking birth certificate. State additions and corrections are incorporated, and a final, national linked file is produced.
Three-year periods were chosen to dampen year-to-year fluctuations. The data were limited to liveborn infants of 20 to 43 weeks' gestation. To reduce implausible birth weight-gestational age combinations, the data were cleaned in the following manner: for each week of gestation, the upper range for birth weight was defined using the gender-specific 97th percentile for singletons41 and the lower limit as the 5th percentile for triplets.42 The data were grouped by weeks of gestation as 20 to 24, 25 to 28, 29 to 32, 33 to 36, 37 to 40, and 41 to 43 weeks. Plurality of each infant was categorized as singletons, twins, triplets, and quadruplets plus quintuplets. Infant mortality was defined as death from 0 to 364 days after birth.
The analyses included several aspects. First, plurality-specific infant mortality rates were calculated overall and by gestation periods for 1989–1991 and 1999–2001 and adjusted odds ratios (AORs) and 95% confidence intervals (CIs) calculated to evaluate the change in risk (improvement in survival) between the 2 time periods. Each of the plurality-specific AORs was then compared with the singleton AORs to evaluate whether the magnitude of change was greater or lesser than the change for singletons during the 2 time periods. Second, these analyses were repeated separately for infants born to white women and to black women. Third, the plurality-specific infant mortality rates were compared for white versus black infants overall and by gestational periods for 1989–1991 and 1999–2001 and AORs and 95% CIs calculated to evaluate the difference in risk between the 2 racial groups during each time period.
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RESULTS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The study population for 1989–1991 and 1999–2001 included 11317895 and 11181095 live births and 89823 and 67129 infant deaths, respectively. Between the 2 time periods, there was a shift to older maternal ages, higher maternal education, more adequacy of prenatal care, and a decrease in smoking during pregnancy overall and for each plurality. Mean length of gestation and birth weight declined significantly overall and for each plurality (Table 1).
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The percent distribution of live births and infant deaths by gestation for plurality and race in 1989–1991 and 1999–2001 are shown in Table 2. Overall, live births were more likely to be delivered earlier at term, at 37 to 40 weeks, and less likely to be delivered at 41 to 43 weeks. Among live births in multiple pregnancies, this trend included a reduction in births at 37 to 40 weeks as well and an increase in the proportion of births at 33 to 36 weeks. Among infant deaths, there was an increase in the proportion born at the border of viability, 20 to 24 weeks, and an overall reduction in the proportion of deaths at later gestational ages. At 20 to 24 weeks, the proportion of live births from multiple versus singleton pregnancies was 10-fold higher and the proportion of infant deaths twice as great.
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The plurality- and gestation-specific infant mortality rates and AORs and 95% CIs, indicating the change in risk of death between 1989–1991 and 1999–2001 are given in Table 3. Overall, the risk of infant death declined significantly for singletons, twins, and triplets but not for quadruplets and quintuplets. For singletons, this improvement was at every gestation; for twins, at births <41 weeks; for triplets, at births <37 weeks; and for quadruplets and quintuplets, only among births between 25 and 28 weeks. Compared with singletons, significantly greater reductions were experienced by twins overall and at gestations <37 weeks and by triplets at gestations <29 weeks.
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These analyses were repeated, limiting the sample to only infants born to white women (Table 4) and only infants born to black women (Table 5). Among white infants, the risk of infant mortality reflected the same pattern as the total population, with significant improvements overall for singletons, twins, and triplets but nonsignificantly for quadruplets and quintuplets. For singletons, this improvement was at every gestation; for twins, for births <41 weeks; for triplets, at births <37 weeks; and for quadruplets and quintuplets, only among births between 25 and 28 weeks. Compared with singletons, significantly greater reductions were experienced by twins overall and at gestations <29 weeks and at 33 to 36 weeks and by triplets at gestations <29 weeks. Among black infants, the risk of infant mortality declined significantly for singletons, overall and at every gestation; for twins, at births <37 weeks; and for triplets, at 25 to 28 weeks. Compared with singletons, only twins born at 25 to 28 weeks had significantly greater improvement in survival, whereas twins overall and those born at 37 to 40 weeks had significantly greater decline in survival.
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A comparison of the plurality- and gestation-specific infant mortality rates and AORs and 95% CIs for white versus black infants within each of the 2 time periods is given in Table 6. In general, the declines in infant mortality rates were greater for white than for black infants. Overall, the racial disparity in risk of infant mortality decreased for singletons and triplets but increased for twins. Within gestational categories, black singleton infants had significantly lower risks before and higher risks after 33 weeks, and black twin infants had significantly lower risks at gestations <25 weeks and higher risk between 33 and 40 weeks. Between 1989–1991 and 1999–2001, this survival advantage at earlier ages diminished substantially and the risk at later gestations increased.
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DISCUSSION |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The results of this study demonstrate the significant reductions in infant mortality that has been achieved during the 1990s. These reductions have been greater for twins and triplets compared with singletons, greater for early preterm (<33 weeks) compared with moderate preterm (33–36 weeks) and term (37–43 weeks) births and for white compared with black infants. The dramatic improvement in survival for infants born at the border of viability (
24 weeks) may represent the widespread use and effectiveness of newer technologies, such as high-frequency oscillatory ventilation, inhaled nitric oxide, and antenatal steroids, but it may also reflect more accurate gestational dating with a greater proportion of births from assisted reproductive technology (ART), particularly to white women. Although national estimates on the use of ART by race are not available, research has indicated racial disparities in outcomes, with black women having higher rates of spontaneous losses and lower live birth rates compared with other racial and ethnic groups.43 This distinction by method of conception is not currently possible; it soon will be with future data based on the 2003 revision of the birth certificate. It is estimated that ARTs contributed 39% to 43% of the increase in triplet and higher-order multiple births in the United States since 1996, with 40% because of ovulation-inducing drugs.1,44 The twin and triplet rates for ART patients are estimated to be 14-fold and 54-fold higher, respectively, than for the United States as a whole.44
Plurality and Racial Disparities in Infant Mortality
The results of this study quantify the improvement in plurality-specific infant mortality rates overall and within gestational periods for all births and for white versus black infants. Although the improvement in survival was greater for twins and triplets compared with singletons, the proportion of deaths among multiples actually increased during the 1990s. The proportion of live births that were multiples increased from 2.4% to 3.2% between the 2 time periods, whereas the proportion of infant deaths increased from 10.6% to 13.6%. Among white infants, multiples accounted for 2.4% and 3.3% of live births and 11.0% and 14.1% of infant deaths in 1989–1991 and 1999–2001, respectively. Among black infants, multiples accounted for 2.7% and 3.5% of live births and 10.7% and 22.2% of infant deaths, respectively, during the 2 time periods. The known survival advantage of black infants at earlier gestations diminished during the 1990s, and their mortality risk at older gestational ages increased. Many reasons may underlie this widening racial disparity in infant survival, including access to appropriate health care and newer technologies, as well as differences in health behaviors. The 2003 report Unequal Treatment by the Institute of Medicine summarized 2 decades of health disparities research.45 Some researchers have suggested that minority children are less likely to benefit from new technologies, such as inhaled steroids, with physicians prescribing them less often than in adults or in nonminority patients.46 This factor of differing access to technology may be less of an issue in obstetric and neonatal care. For example, Hamvas et al,29 in their study of surfactant therapy on neonatal mortality among blacks and whites, found that the technology resulted in greater reductions in mortality for white infants and that the difference was not explained by access to surfactant or antenatal corticosteroid therapies. Maternal and family health behaviors may also contribute to this increased risk, such as infant sleeping practices. Positioning infants in a prone sleeping position, associated with one third of all SIDS deaths, is more common among black families,47,48 despite the success of the American Academy of Pediatrics national Back to Sleep campaign during the 1990s.49 In their analysis of racial differences in infant mortality, Muhuri et al50 reported that non-Hispanic blacks had more than a twofold excess odds for 3 causes of infant mortality (RDS, short gestation/low birth weight, and maternal complications) compared with non-Hispanic whites, which are closely related to their twofold greater risk of delivering a low birth-weight infant.
Birth certificate data provide our only national perinatal database on the outcome of pregnancy in the United States. When linked with the infant death records, this information provides important insights into relationships between perinatal factors and the timing and causes of infant death. The use of vital statistics data has limitations.51–55 First, there is the potential for misclassification of gestational age. We have attempted to minimize this error by cleaning the data to eliminate implausible birth weight-gestational age combinations, but it is still possible that some remain despite this effort. Second, because of the intracluster correlation of mortality risk for infants born in a multiple versus singleton pregnancy, the risk of death for these infants is acknowledged to be greater, beyond that because of lowered birth weight and younger gestational age.56,57 The annual linked birth and death files are reported as individual births, although NCHS has released a matched multiple birth file that links multiple births to the same mother; this matched file is only available for births between 1995 and 2000, which includes only a portion of the time period evaluated in the current study. Despite these limitations, the use of multiyear, population-based data provides the best available data for evaluating national trends in live births and infant deaths.
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CONCLUSIONS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Infant mortality rates have declined substantially during the 1990s, although the racial disparity in survival widened. Multiple births contribute disproportionately to infant mortality in large part because of their tendency to be born prematurely. The improved survival of smaller and more immature infants has long-term social, economic, and health implications.
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ACKNOWLEDGMENTS |
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This study was supported by grants RO3 HD048498 and RO3 HD047627 from National Institute of Child Health and Human Development, National Institutes of Health.
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FOOTNOTES |
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Accepted Aug 7, 2006.
Address correspondence to Barbara Luke, ScD, MPH, RN, RD, School of Nursing and Health Studies, University of Miami, 5801 Red Rd, Coral Gables, FL. E-mail: bluke{at}med.miami.edu
This work was presented at the annual meeting of the Society for Pediatric and Perinatal Epidemiology; June 20-21, 2006; Seattle, WA.
The authors have indicated they have no financial relationships relevant to this article to disclose.
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PEDIATRICS (ISSN 1098-4275). ©2006 by the American Academy of Pediatrics
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