Marian F. MacDorman

The Contribution of Preterm Birth to Infant Mortality Rates in the United States

OBJECTIVE. Although two thirds of infant deaths in the United States occur among infants born preterm (<37 weeks of gestation), only 17% of infant deaths are classified as being attributable to preterm birth with the standard classification of leading causes of death. To address this apparent discrepancy, we sought to estimate more accurately the contribution of preterm birth to infant mortality rates in the United States. METHODS. We identified the top 20 leading causes of infant death in 2002 in the US linked birth/infant death file. The role of preterm birth for each cause was assessed by determining the proportion of infants who were born preterm for each cause of death and by considering the biological connection between preterm birth and the specific cause of death. RESULTS. Of 27970 records in the linked birth/infant death file for 2002, the 20 leading causes accounted for 22273 deaths (80% of all infant deaths). Among infant deaths attributable to the 20 leading causes, we classified 9596 infant deaths (34.3% of all infant deaths) as attributable to preterm birth. Ninety-five percent of those deaths occurred among infants who were born at <32 weeks of gestation and weighed <1500 g, and two thirds of those deaths occurred during the first 24 hours of life. CONCLUSIONS. On the basis of this evaluation, preterm birth is the most frequent cause of infant death in the United States, accounting for at least one third of infant deaths in 2002. The extreme prematurity of most of the infants and their short survival indicate that reducing infant mortality rates requires a comprehensive agenda to identify, to test, and to implement effective strategies for the prevention of preterm birth.

Annual Summary of Vital Statistics—2003

The crude birth rate rose slightly in 2003 to 14.1 births per 1000 population, from 13.9 in 2002. The 2002 rate was the lowest ever reported for the United States. The total number of births and the fertility rate (66.1) also increased. The birth rate for teenaged mothers dropped 3% to another record low in 2003, to 41.7 per 1000 females aged 15 to 19 years. The teenage birth rate has fallen by one third since 1991. The birth rate declined for women 20 to 24 years old but rose for women aged 25 to 44 years. The number, rate, and proportion of births to unmarried women all increased in 2003. Smoking during pregnancy declined to 11%, down from 19.5% in 1989. Prenatal care utilization improved slightly for 2003; 84.1% of women began care in the first trimester of pregnancy. The cesarean delivery rate jumped 6% to 27.6% for another US high. The primary cesarean rate rose 6%, and the rate of vaginal birth after a previous cesarean delivery plummeted 16% from 2002 to 2003. The percent of infants delivered preterm continued to rise (12.3% in 2003). The preterm birth rate is up 16% since 1990. The percentage of children born at low birth weight rose slightly in 2003 to the highest level reported since 1970 (7.9%). The twinning rate increased, but the rate for triplet/+ births declined slightly between 2001 and 2002. Multiple births accounted for 3.3% of all births in 2002. The infant mortality rate rose to 7.0/1000 live births in 2002 from 6.8 in 2001, marking the first increase in this rate in >4 decades. Increases were distributed fairly widely across age, racial/ethnic groups, and geographic areas. The rise in infant mortality was attributed to increases in <750-g births in both singleton and multiple deliveries. Although the downward trend in infant mortality rates in many developed nations may have stabilized, the United States still ranked 27th among these nations in 2001. Expectation of life at birth reached a record high of 77.3 years for all gender and race groups combined in 2002. Death rates in the United States continue to decline. Between 2001 and 2002, death rates declined for the 3 leading causes of death: diseases of heart, malignant neoplasms, and cerebrovascular diseases. Death rates for children 1 to 19 years old decreased by 8% for suicide; the death rate for chronic lower respiratory diseases increased by 33% in 2002. Rates for unintentional injuries and homicide did not change significantly for children aged 1 to 19 years. A large proportion of childhood deaths continues to occur as a result of preventable injuries.

Changes in the classification of sudden unexpected infant deaths: United States, 1992-2001

Background. Sudden infant death syndrome (SIDS) makes up the largest component of sudden unexpected infant death in the United States. Since the first recommendations for supine placement of infants to prevent SIDS in 1992, SIDS postneonatal mortality rates declined 55% between 1992 and 2001. Objective. The objective of this analysis was to examine changes in postneonatal mortality rates from 1992 to 2001 to determine if the decline in SIDS was due in part to a shift in certification of deaths from SIDS to other causes of sudden unexpected infant death. In addition, the analysis reviews the change in mortality rates attributed to the broad category of sudden unexpected infant death in the United States since 1950. Methods. US mortality data were used. The International Classification of Diseases (ICD) chapters “Symptoms, Signs, and Ill-Defined Conditions” and “External Causes of Injury” were considered to contain all causes of sudden unexpected infant death. The following specific ICD (ninth and tenth revisions) underlying-cause-of-death categories were examined: “SIDS,” “other unknown and unspecified causes,” “suffocation in bed,” “suffocation-other,” “aspiration,” “homicide,” and “injury by undetermined intent.” The average annual percentage change in rates was determined by Poisson regression. An analysis was performed that adjusted mortality rates for changes in classification between ICD revisions. Results. The all-cause postneonatal mortality rate declined 27% and the postneonatal SIDS rate declined 55% between 1992 and 2001. However, for the period from 1999 to 2001 there was no significant change in the overall postneonatal mortality rate, whereas the postneonatal SIDS rate declined by 17.4%. Concurrent increases in postneonatal mortality rates for unknown and unspecified causes and suffocation account for 90% of the decrease in the SIDS rate between 1999 and 2001. Conclusions. The failure of the overall postneonatal mortality rate to decline in the face of a declining SIDS rate in 1999–2001 raises the question of whether the falling SIDS rate is a result of changes in certifier practices such that deaths that in previous years might have been certified as SIDS are now certified to other non-SIDS causes. The observation that the increase in the rates of non-SIDS causes of sudden unexpected infant death could account for >90% of the drop in the SIDS rates suggests that a change in classification may be occurring.

Fetal and neonatal mortality among twin gestations in the United States: the role of intrapair birth weight discordance

OBJECTIVE To examine the association of intrapair birth weight discordance with fetal and neonatal mortality. METHODS We used the United States (1995–1997) Matched Multiple Birth File (n = 297,155). RESULTS Among twin live births and stillborn fetuses, 29.9% had less than 5% birth weight discordance, 24.2% had 5–9%, 29.6% had 10–19%, 11.1% had 20–29%, 3.4% had 30–39%, and 1.8% had 40% or more. The stillborn fetus rate increased progressively with increasing birth weight discordance for smaller and larger twins of the same sex. Compared with the less than 5% birth weight discordance category, the adjusted odds ratios (OR) (95% confidence intervals [CIs]) for stillborn fetus associated with 5–9%, 10–19%, 20–29%, 30–39%, and 40% or more birth weight discordance, respectively, were 0.81 (95% CI 0.58, 1.11), 1.41 (95% CI 1.07, 1.84), 1.74 (95% CI 1.28, 2.35), 3.06 (95% CI 2.21, 4.24), and 4.29 (95% CI 3.05, 6.04) for smaller twins. The corresponding ORs (95% CIs) for larger twins were 0.78 (95% CI 0.57, 1.08), 1.26 (95% CI 0.96, 1.66), 1.77 (95% CI 1.27, 2.46), 3.38 (95% CI 2.33, 4.92), and 2.91 (95% CI 1.89, 4.47). Similar associations were observed among smaller but not larger twins of opposite sex. Among larger but not smaller twins of the same sex, increasing birth weight discordance was associated with overall neonatal deaths. This association was not apparent among smaller and larger twins of opposite sex. However, increasing birth weight discordance was associated with neonatal deaths related to congenital malformations among smaller and larger twins. CONCLUSION The results provide evidence that increased twin birth weight discordance was associated with increased risk of intrauterine death and malformation‐related neonatal deaths.

Race and Ethnic Disparities in Fetal Mortality, Preterm Birth, and Infant Mortality in the United States: An Overview

Infant mortality, fetal mortality, and preterm birth all represent important health challenges that have shown little recent improvement. The rate of decrease in both fetal and infant mortality has slowed in recent years, with little decrease since 2000 for infant mortality, and no significant decrease from 2003 to 2005 for fetal mortality. The percentage of preterm births increased by 36% from 1984 to 2006, and then decreased by 4% from 2006 to 2008. There are substantial race and ethnic disparities in fetal and infant mortality and preterm birth, with non-Hispanic black women at greatest risk of unfavorable birth outcomes, followed by American Indian and Puerto Rican women. Infant mortality, fetal mortality, and preterm birth are multifactorial and interrelated problems with similarities in etiology, risk factors and disease pathways. Preterm birth prevention is critical to lowering the infant mortality rate, and to reducing race and ethnic disparities in infant mortality.

Recent Trends and Patterns in Cesarean and Vaginal Birth After Cesarean (VBAC) Deliveries in the United States

Cesarean delivery is the most common major surgical procedure for women in the United States, with 1.4 million surgeries annually. In 2008, nearly one-third (32.3%) of US births were by cesarean delivery. Cesarean delivery rates have increased rapidly in the United States in recent years because of an increasing primary cesarean delivery rate and a declining vaginal birth after cesarean (VBAC) rate. In 2007, the VBAC rate was 8.3% in a 22-state reporting area. The US VBAC rate was lowest among 14 industrialized countries; 3 countries had VBAC rates greater than 50%.

Annual Summary of Vital Statistics: 2012–2013

The number of births in the United States declined by 1% between 2012 and 2013, to a total of 3 932 181. The general fertility rate also declined 1% to 62.5 births per 1000 women, the lowest rate ever reported. The total fertility rate was down by 1% in 2013 (to 1857.5 births per 1000 women). The teenage birth rate fell to another historic low in 2013, 26.5 births per 1000 women. Birth rates also declined for women 20 to 29 years, but the rates rose for women 30 to 39 and were unchanged for women 40 to 44. The percentage of all births that were to unmarried women declined slightly to 40.6% in 2013, from 40.7% in 2012. In 2013, the cesarean delivery rate declined to 32.7% from 32.8% for 2012. The preterm birth rate declined for the seventh straight year in 2013 to 11.39%; the low birth weight (LBW) rate was essentially unchanged at 8.02%. The infant mortality rate was 5.96 infant deaths per 1000 live births in 2013, down 13% from 2005 (6.86). The age-adjusted death rate for 2013 was 7.3 deaths per 1000 population, unchanged from 2012. Crude death rates for children aged 1 to 19 years declined to 24.0 per 100 000 population in 2013, from 24.8 in 2012. Unintentional injuries and suicide were, respectively, the first and second leading causes of death in this age group. These 2 causes of death jointly accounted for 45.7% of all deaths to children and adolescents in 2013.

Obstetrical intervention and the singleton preterm birth rate in the United States from 1991-2006.

OBJECTIVES We examined the relationship between obstetrical intervention and preterm birth in the United States between 1991 and 2006. METHODS We assessed changes in preterm birth, cesarean delivery, labor induction, and associated risks. Logistic regression modeled the odds of preterm obstetrical intervention after risk adjustment. RESULTS From 1991 to 2006, the percentage of singleton preterm births increased 13%. The cesarean delivery rate for singleton preterm births increased 47%, and the rate of induced labor doubled. In 2006, 51% of singleton preterm births were spontaneous vaginal deliveries, compared with 69% in 1991. After adjustment for demographic and medical risks, the mother of a preterm infant was 88% (95% confidence interval [CI] = 1.87, 1.90) more likely to have an obstetrical intervention in 2006 than in 1991. Using new birth certificate data from 19 states, we estimated that 42% of singleton preterm infants were delivered via induction or cesarean birth without spontaneous onset of labor. CONCLUSIONS Obstetrical interventions were related to the increase in the US preterm birth rate between 1991 and 2006. The public health community can play a central role in reducing medically unnecessary interventions.

Recent Increases in the U.s. Maternal Mortality Rate: Disentangling Trends From Measurement Issues

OBJECTIVE: To develop methods for trend analysis of vital statistics maternal mortality data, taking into account changes in pregnancy question formats over time and between states, and to provide an overview of U.S. maternal mortality trends from 2000 to 2014. METHODS: This observational study analyzed vital statistics maternal mortality data from all U.S. states in relation to the format and year of adoption of the pregnancy question. Correction factors were developed to adjust data from before the standard pregnancy question was adopted to promote accurate trend analysis. Joinpoint regression was used to analyze trends for groups of states with similar pregnancy questions. RESULTS: The estimated maternal mortality rate (per 100,000 live births) for 48 states and Washington, DC (excluding California and Texas, analyzed separately) increased by 26.6%, from 18.8 in 2000 to 23.8 in 2014. California showed a declining trend, whereas Texas had a sudden increase in 2011–2012. Analysis of the measurement change suggests that U.S. rates in the early 2000s were higher than previously reported. CONCLUSION: Despite the United Nations Millennium Development Goal for a 75% reduction in maternal mortality by 2015, the estimated maternal mortality rate for 48 states and Washington, DC, increased from 2000 to 2014; the international trend was in the opposite direction. There is a need to redouble efforts to prevent maternal deaths and improve maternity care for the 4 million U.S. women giving birth each year.Background A pregnancy question was added to the U.S. standard death certificate in 2003 to improve ascertainment of maternal deaths. The delayed adoption of this question among states led to data incompatibilities, and impeded accurate trend analysis. Our objectives were to develop methods for trend analysis, and to provide an overview of U.S. maternal mortality trends from 2000–2014.

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.