Explaining the 2001-02 infant mortality increase: data from the linked birth/infant death data set.

The U.S. infant mortality rate increased from 6.8 infant deaths per 1,000 live births in 2001 to 7.0 in 2002, the first increase in more than 40 years. From 2001 to 2002 infant mortality rates increased for very low birthweight infants as well as for preterm and very preterm infants. Although infant mortality rates for very low birthweight infants increased, most of the increase in the infant mortality rate from 2001 to 2002 was due to a change in the distribution of births by birthweight and, more specifically, to an increase in infants born weighing less than 750 grams (1 lb 10 1/2 oz). The majority of infants born weighing less than 750 grams die within the first year of life; thus, these births contribute disproportionately to the overall infant mortality rate. Increases in births at less than 750 grams occurred for non-Hispanic white, non-Hispanic black, and Hispanic women. Most of the increase occurred among mothers 20-34 years of age. Although multiple births contributed disproportionately, most of the increase in births at less than 750 grams occurred among singletons. Three hypotheses were evaluated to assess their possible impact on the increase in less than 750-gram births: first, possible changes in the reporting of births or fetal deaths; second, possible changes in the risk profile of births; and third, possible changes in medical management of pregnancy. Although each of these factors may have contributed to the increase, the relative effects of these and other factors remain unclear. More-detailed studies are needed to further explain the 2001-02 infant mortality increase.     

Trends in preterm-related infant mortality by race and ethnicity, United States, 1999-2004.

Trends in preterm-related causes of death were examined by maternal race and ethnicity. A grouping of preterm-related causes of infant death was created by identifying causes that were a direct cause or consequence of preterm birth. Cause-of-death categories were considered to be preterm-related when 75 percent or more of total infant deaths attributed to that cause were deaths of infants born preterm, and the cause was considered to be a direct consequence of preterm birth based on a clinical evaluation and review of the literature. In 2004, 36.5 percent of all infant deaths in the United States were preterm-related, up from 35.4 percent in 1999. The preterm-related infant mortality rate for non-Hispanic black mothers was 3.5 times higher and the rate for Puerto Rican mothers was 75 percent higher than for non-Hispanic white mothers. The preterm-related infant mortality rate for non-Hispanic black mothers was higher than the total infant mortality rate for non-Hispanic white, Mexican, and Asian or Pacific Islander mothers. The leveling off of the U.S. infant mortality decline since 2000 has been attributed in part to an increase in preterm and low-birthweight births. Continued tracking of preterm-related causes of infant death will improve our understanding of trends in infant mortality in the United States.     

Explaining the 2001-2002 infant mortality increase in the United States: data from the linked birth/infant death data set.

The U.S. infant mortality rate (IMR) increased from 6.8 infant deaths per 1,000 live births in 2001 to 7.0 in 2002, the first increase in more than 40 years. From 2001 to 2002, IMR increased for very low birthweight infants as well as for preterm and very preterm infants. Although IMR for very low birthweight infants increased, most of the increase in IMR from 2001 to 2002 was due to a change in the distribution of births by birthweight and, more specifically, to an increase in infants born weighing less than 750 grams. The majority of infants born at less than 750 grams die within the first year of life; thus, these births contribute disproportionately to overall IMR. Increases in births at less than 750 grams occurred fornon-Hispanic white, non-Hispanic black, and Hispanic women. Most of the increase occurred among mothers 20 to 34 years of age. Although multiple births contributed disproportionately, most of the increase in births at less than 750 grams occurred among singletons. Three hypotheses were evaluated to assess their possible impact on the increase in less than 750-gram births: possible changes in (1) the reporting of births or fetal deaths, (2) the risk profile of births, and (3) medical management of pregnancy. Although each of these factors may have contributed to the increase, the relative effects of these and other factors remain unclear. More detailed studies are needed to further explain the 2001-2002 infant mortality increase.     

Mortality risk of small infants varies with their mother's birthweight and race

We analysed a transgenerational linked birth file to investigate the relationship between maternal birthweight and infant birthweight-specific mortality risk for white and African American infants. Birth records of 267,303 infants born between 1989 and 1991 were linked to records of their mothers, born between 1956 and 1976, and to their own death certificates for those dying in the first year. The means, standard deviations and z-scores were calculated for each race- and generation-specific birthweight distribution. Investigators then analysed the mortality of very small infants (birthweight at least two standard deviations below their mean) for three maternal birthweight categories. Over half of the infant deaths involved births with weights more than two standard deviations below the relevant population mean birthweight (comprising 4.2% of white and 6.9% of African American births respectively). African American infants experienced higher mortality rates at all levels of standardised birthweight, from z-scores of -3 to +3. The relative risk of mortality associated with very small infant size was less for infants delivered to smaller birthweight mothers when compared with those whose mothers were average sized or large at birth. This differential effect was confined to neonatal deaths and was more prominent in the white subpopulation.     

Trends in reporting of maternal drug abuse and infant mortality among drug-exposed infants in New York City

New York City trends in maternal drug abuse during pregnancy and in mortality rates for infants with in utero drug exposure are reported; causes of death among drug-exposed infants are studied, as is the association between maternal drug abuse and other factors that contribute to infant mortality (e.g., low birthweight, lack of prenatal care). Data for this study are derived from the linked files of New York City birth and infant death certificates. Reports of infants born to drug abusing mothers increased from 6.7 per 1000 live births in 1981 to 20.3 per 1000 live births in 1987, with abuse of cocaine accounting for most of the rise. When standardized for race and ethnicity, the mortality rate for drug-exposed infants born from 1978 through 1986 was 35.9, or 2.4 times that for infants in New York City in general. Drug-exposed infants were over three times as likely as infants in the general population to be of low birthweight. The association of both opiates and cocaine with increased mortality and low birthweight was similar. Death rates from SIDS and AIDS were especially higher for drug-exposed infants than for those in the general population, and were similar for opiate- and cocaine-exposed infants. The impact of drug abuse on infant mortality rates in selected low socioeconomic health districts is discussed.     

Disparities in Mortality Rates Among US Infants Born Late Preterm or Early Term, 2003–2005

The purpose of this study was to identify disparities in neonatal, post-neonatal, and overall infant mortality rates among infants born late preterm (34–36 weeks gestation) and early term (37–38 weeks gestation) by race/ethnicity, maternal age, and plurality. In analyses of 2003–2005 data from US period linked birth/infant death datasets, we compared infant mortality rates by race/ethnicity, maternal age, and plurality among infants born late preterm or early term and also determined the leading causes of death among these infants. Among infants born late preterm, infants born to American Indian/Alaskan Native, non-Hispanic black, or teenage mothers had the highest infant mortality rates per 1,000 live births (14.85, 9.90, and 11.88 respectively). Among infants born early term, corresponding mortality rates were 5.69, 4.49, and 4.82, respectively. Among infants born late preterm, singletons had a higher infant mortality rate than twins (8.59 vs. 5.62), whereas among infants born early term, the rate was higher among twins (3.67 vs. 3.15). Congenital malformations and sudden infant death syndrome were the leading causes of death among both late preterm and early term infants. Infant mortality rates among infants born late preterm or early term varied substantially by maternal race/ethnicity, maternal age, and plurality. Information about these disparities may help in the development of clinical practice and prevention strategies targeting infants at highest risk.     

The intergenerational relationship between mother's birthweight, infant birthweight and infant mortality in black and white mothers

Summary. The relationship between the birthweight of white and black mothers and the outcomes of their infants were examined using the 1988 National Maternal and Infant Health Survey. White and black women who were low birthweight themselves were at increased risk of delivering very low birthweight (VLBW), moderately low birthweight (MLBW), extremely preterm and small size for gestational age (SGA) infants. Adjustment for the confounding effects of prepregnant weight and height reduced the risks of all these outcomes slightly, and more substantially reduced the maternal birthweight associated risk of moderately low birthweight among white mothers. There was little effect of maternal birthweight on infant birthweight-specific infant mortality in white mothers; however, black mothers who weighed less than 4 lbs at birth were at significantly increased risk of delivering a normal birthweight infant who subsequently died. Although the risks for the various outcomes associated with low maternal birthweight were not consistently higher in black mothers compared with white mothers, adjustment for prepregnant weight and height had a greater effect in white mothers than in black mothers. We suggest that interventions to reduce the risks for adverse pregnancy outcomes associated with low maternal birthweight should attempt to optimise prepregnant weight and foster child health and growth.     

Perinatal Periods of Risk: Analysis of Fetal–Infant Mortality Rates in Kansas City,Missouri

Objectives: The Perinatal Periods of Risk (PPOR) technique was used to analyze resident fetal and infant death data from Kansas City, Missouri, for the period 1998–2002. Results offer important information that can be used to develop community-based prevention strategies related to racial/ethnic disparities in infant mortality rates (IMR). Methods: The PPOR approach for fetal and infant mortality can be mapped by birthweight at delivery and time of death into four strategic prevention areas: 1) Maternal Health/Prematurity (MHP), 2) Maternal Care (MC), 3) Newborn Care (NC), and 4) Infant Health (IH). For this analysis, all fetal and infant death certificates from the metropolitan Kansas City area were linked to their birth certificates and those associated with residents of Kansas City, Missouri, proper were used to create the dataset used in this analysis. Due to the small number of fetal and infant deaths among other ethnic groups, the analysis was restricted to a comparison of the disparity of IMR between Blacks, Whites, and a national non-Hispanic white reference group. The Kitagawa formula was used to determine contribution to excess deaths from birthweight-specific mortality and birthweight distribution rates. Logistic regression techniques were used to identify risk factors for death among Black fetuses and infants with very low birthweights and also deaths due to sudden infant death syndrome (SIDS). Results: The PPOR analysis showed that of the excess deaths among black infants, when compared to a national reference group, 47% was attributable to MHP and another 29% was attributable to IH. Differences in MC and NC only accounted for 27 and 8% of the total excess deaths. During the study period, rates of sudden infant death syndrome (SIDS) were found to be significantly higher among Blacks as compared to Whites (2.12 vs. 0.81 per 1,000). An analysis of maternal characteristics for SIDS deaths among blacks using a step-wise logistic regression model, found that maternal age less than 20 years old, previous births, inadequate prenatal care, and being a Medicaid recipient were significant—adjusted odds ratios of 23.7 (95% Cl 10.48, 53.67), 8.4 (95% Cl 3.64, 19.21), 2.9 (95% Cl 1.38, 6.05) and 2.5 (95% Cl 1.04, 5.84), respectively. Conclusions: PPOR is an easy to use approach that helps focus community initiatives for improving maternal and infant health. In Kansas City, Missouri, efforts to further lower IMR in blacks can be achieved through the reduction of risk factors affecting maternal health and through maternal education to improve infant health.     

Impact of very low birthweight on the black-white infant mortality gap.

In recent years, the rate of decline for the black infant mortality risk (IMR) has been slower than that for whites. The resultant widening in the black-white infant mortality gap has been accompanied by an increased percentage of very low birthweight (VLBW) infants (227 g-1,499 g) among black live births. Restricting our analysis to non-Hispanic black and white single live births, we used the 1983 national linked birth-death file to assess the relative contribution of VLBW infants to the black-white gap in IMR. VLBW occurred among 2.3% of all black live births and among 0.8% of all white live births. Deaths among VLBW infants accounted for 62.5% of the black-white gap in IMR. Although VLBW newborns represent a fraction of all live births in the United States, they account for almost two-thirds of the black-white gap in IMR. Since preterm delivery is associated with most VLBW infant deaths, our findings indicate the crucial need to identify strategies that reduce preterm births, among blacks in particular, to reduce significantly the infant mortality gap in the United States.     

Secular trends of sudden infant death syndrome in Norway 1967–1988: application of a method of case identification to Norwegian registry data

Summary. In Norway, towards the end of the 1980s, sudden infant death syndrome (SIDS) was the most frequent cause of infant death. Both SIDS and the total post-perinatal mortality rates had increased. This paper presents a procedure for identifying SIDS from death certificates. Supplemented with additional information, a database was established to evaluate secular trends of SIDS and for further analytical research. The Medical Birth Registry of Norway comprises 1.3 million births from 1967 to 1988. Of these, 5447 infants died in the post-perinatal period. The cause of death was reviewed by an expert panel and 1984 cases of SIDS were retrieved.
Low maternal age, higher birth order, male gender, and lower birthweight were confirmed as risk factors for SIDS. In 1988, the rate for SIDS and for total post-perinatal deaths reached 2.69 and 5.02 per 1000 infants at risk. The incidence of SIDS increased 2.2 times from the period 1967–1971 to the period 1987–1988. Adjusted for maternal age, birth order, and birthweight, the odds ratio was 3.1. The increase is due to factors not yet accounted for. Adjusted mortality rates for the other post-perinatal deaths were not different from the crude rates.     

Racial differences in leading causes of infant death in the United States

We used linked birth/infant death records of over 23 million singletons belonging to six birth cohorts (1989-91 and 1995-97) and examined changes in race differentials in the overall and cause-specific infant mortality risks across time in the United States. Results show that infant mortality declined for all races during the time period, with disproportionately greater declines among non-Hispanic American Indians (AIs). Among the leading causes of infant death, declines in mortality from sudden infant death syndrome (SIDS), respiratory distress syndrome (RDS) and congenital anomalies contributed the most to the overall decline in infant mortality in the 1995-97 cohorts, compared with the 1989-91 cohorts. Disproportionately greater reductions in mortality resulting from SIDS and congenital anomalies led to more rapid mortality declines among non-Hispanic AIs than for other races. There are disturbing findings that infants of almost every race experienced increases in mortality from newborn affected by maternal complications of pregnancy (maternal complications) and that none of the race groups experienced a significant decline in mortality from disorders resulting from short gestation/low birthweight.     

Trends in twin and triplet births: 1980-97.

This report presents data from U.S. birth certificates on the numbers and rates of twin and triplet and other higher order multiple births for 1980-97. Over the study period, the number of twin births rose 52 percent (from 68,339 to 104,137) and triplet and other higher order multiple births (heretofore referred to as "triplet/+") climbed 404 percent (from 1,337 to 6,737 births). Comparable but less pronounced rises were observed in twin and triplet/+ birth rates. Growth in twin and triplet/+ birth rates was most marked among women aged 30 years and over. Between 1980-82 and 1995-97, the twin rate rose 63 percent for women aged 40-44 years, and soared nearly 1,000 percent for women 45-49 years. (As one result, there were more twins born to women 45-49 years of age in 1997, than during the entire decade of the 1980's.) The triplet/+ birth rate rose nearly 400 percent for women in their thirties and exploded by more than 1,000 percent for women in their forties. The extraordinary rise in multiple births resulted in a shift in age-specific patterns, and the highest twin and triplet/+ birth rates now are for women 45-49 years of age. Historical differences in twinning rates between non-Hispanic white and black mothers have been largely eliminated (28.8 per 1,000 non-Hispanic white compared with 30.0 for black women). Non-Hispanic white women were more than twice as likely as non-Hispanic black or Hispanic women to have a triplet/+ birth. Rates of low birthweight, very low birthweight, and infant mortality were 4 to 33 times higher for twins and triplet/+ compared with singleton births. The risk for these adverse outcomes was lowest for twins and triplet/+ born to women 35-44 years of age. Twin birth rates for Massachusetts and Connecticut were at least 25 percent higher than the U.S. rate; triplet/+ rates for Nebraska and New Jersey were twice the national level.     

Overview of the National Infant Mortality Surveillance (NIMS) project

A slowdown in the decline of infant mortality in the United States and a continuing high risk of death among black infants (twice that of white infants) prompted a consortium of Public Health Service agencies, in collaboration with all states, to develop a national data base of linked birth and infant death certificates for the 1980 birth cohort. This project, referred to as National Infant Mortality Surveillance (NIMS), provides neonatal, postneonatal, and infant mortality risks for blacks, whites, and all races in 12 categories of birthweights. Tabulations were requested for infants born in single and multiple deliveries. For single-delivery births, tabulations included birthweight, age at death, race of infant, and each of these characteristics: infant's live-birth order, sex, gestation, type of delivery, and cause of death; and mother's age, education, prenatal care history, and number of prior fetal losses at greater than or equal to 20 weeks' gestation. An estimated 95% of eligible infant deaths were included in the NIMS tabulations. Analyses have focused on various components of infant mortality, including birthweight distribution of live births, neonatal mortality, and postneonatal mortality. The most important predictor for infant survival is birthweight; survival increases exponentially as birthweight increases to its optimal level. The nearly twofold higher risk of infant mortality among blacks than among whites was related to a higher prevalence of low birthweights, to higher mortality risks in the neonatal period for infants with birthweights of greater than or equal to 3,000 g, and to higher mortality during the postneonatal period for all infants, regardless of birthweight. Moreover, the black-white gap persisted for infants with birthweight of greater than or equal to 2,500 g, regardless of other infant or maternal risk factors.     

Developing a Maternally Linked Birth Dataset to Study the Generational Recurrence of Low Birthweight in Virginia

This paper examined the generational recurrence of low birthweight (LBW) among first-born singletons using a statewide maternally-linked birth dataset. An intergenerational dataset was created by linking 2005–2009 to 1960–1997 Virginia resident live birth data. Maternal information from the recent birth cohort was linked to infant information in the historic birth file using various combinations of mother’s name and birthdate. The linked dataset contained 170,624 records (87 % of all eligible records). The analysis dataset was limited to non-Hispanic black and non-Hispanic white first-born singleton infants linked to their mother’s own birth record (n = 69,702). Maternal birthweight was a significant predictor of LBW for first-born singletons. The birthweight distribution for both non-Hispanic black and non-Hispanic white infants was shifted toward lower birthweights for infants whose mothers were born LBW. Even after adjusting for known maternal risk factors in the current pregnancy, non-Hispanic black (AOR = 1.6 [95 % CI 1.4, 1.8]) and non-Hispanic white (AOR = 2.0 [95 % CI 1.8, 2.3]) infants had increased odds of being born LBW if their mother was born LBW. A mother’s early life experiences can impact the health of her children. These findings underscore the importance of applying a life course perspective to the prevention of LBW. Routine linkage of maternal and infant birth data is needed to strengthen the evidence base for policies and programs that address issues affecting maternal and child health throughout the life course.     

Surveillance of postneonatal mortality, United States, 1980-1987

In the United States, one-third of all infant deaths (deaths of infants ages 0-364 days) occurs in the postneonatal period (28-364 days). A substantial proportion of these deaths potentially could be prevented. To examine recent trends in postneonatal mortality (PNM) in the United States, the investigators analyzed birth and death certificate data for resident infants for the period from 1980 through 1987. Rates of PNM declined 11% from 3.5 to 3.1/1,000 live births among white infants and declined 16% from 7.3 to 6.1/1,000 live births among black infants. Most of the decline resulted from reduced mortality from infectious diseases and injuries. A decreased mortality attributable to sudden infant death syndrome (SIDS) among black infants additionally accounted for the decline. Autopsy rates for SIDS increased from 82% to 92% but did not differ for black infants and white infants. Birth defects-related PNM declined more among white infants than among black infants. The racial gap in PNM (rate ratio (RR) = approximately 2.0) persisted. However, the largest black/white gap occurred in the Northeast (RR = 2.5), the region with the lowest PNM. Black infants were 2.7 and 2.3 times more likely to die of infections and injuries, respectively, than were white infants. Although PNM rates declined during the 1980s, a greater rate of reduction is needed to achieve the Year 2000 objectives, especially among black infants. Such reductions are possible through improved access to comprehensive pediatric care as well as education and community-oriented prevention programs designed to reduce deaths due to infections and injuries. A better understanding of the etiology of SIDS and birth defects is critical for preventing postneonatal deaths.