Causes of death and infant mortality rates among full-term births in the United States between 2010 and 2012: An observational study

Abstract

Background: While the high prevalence of preterm births and its impact on infant mortality in the US have been widely acknowledged, recent data suggest that even full-term births in the US face substantially higher mortality risks compared to European countries with low infant mortality rates. In this paper, we use the most recent birth records in the US to more closely analyze the primary causes underlying mortality rates among full-term births.

Methods and findings: Linked birth and death records for the period 2010-2012 were used to identify the state- and cause-specific burden of infant mortality among full-term infants (born at 37-42 weeks of gestation). Multivariable logistic models were used to assess the extent to which state-level differences in full-term infant mortality (FTIM) were attributable to observed differences in maternal and birth characteristics. Random effects models were used to assess the relative contribution of state-level variation to FTIM. Hypothetical mortality outcomes were computed under the assumption that all states could achieve the survival rates of the best-performing states. A total of 10,175,481 infants born full-term in the US between January 1, 2010, and December 31, 2012, were analyzed. FTIM rate (FTIMR) was 2.2 per 1,000 live births overall, and ranged between 1.29 (Connecticut, 95% CI 1.08, 1.53) and 3.77 (Mississippi, 95% CI 3.39, 4.19) at the state level. Zero states reached the rates reported in the 6 low-mortality European countries analyzed (FTIMR < 1.25), and 13 states had FTIMR > 2.75. Sudden unexpected death in infancy (SUDI) accounted for 43% of FTIM; congenital malformations and perinatal conditions accounted for 31% and 11.3% of FTIM, respectively. The largest mortality differentials between states with good and states with poor FTIMR were found for SUDI, with particularly large risk differentials for deaths due to sudden infant death syndrome (SIDS) (odds ratio [OR] 2.52, 95% CI 1.86, 3.42) and suffocation (OR 4.40, 95% CI 3.71, 5.21). Even though these mortality differences were partially explained by state-level differences in maternal education, race, and maternal health, substantial state-level variation in infant mortality remained in fully adjusted models (SIDS OR 1.45, suffocation OR 2.92). The extent to which these state differentials are due to differential antenatal care standards as well as differential access to health services could not be determined due to data limitations. Overall, our estimates suggest that infant mortality could be reduced by 4,003 deaths (95% CI 2,284, 5,587) annually if all states were to achieve the mortality levels of the best-performing state in each cause-of-death category. Key limitations of the analysis are that information on termination rates at the state level was not available, and that causes of deaths may have been coded differentially across states.

Conclusions: More than 7,000 full-term infants die in the US each year. The results presented in this paper suggest that a substantial share of these deaths may be preventable. Potential improvements seem particularly large for SUDI, where very low rates have been achieved in a few states while average mortality rates remain high in most other areas. Given the high mortality burden due to SIDS and suffocation, policy efforts to promote compliance with recommended sleeping arrangements could be an effective first step in this direction.

Fig 1. Relative mortality risk in the US and Europe by gestational age category.

The figure shows infant mortality risk (IMR) in the US compared to the average rate observed in Austria, Denmark, Finland, Norway, Sweden, and Switzerland for the year 2010. Sources: Euro-Peristat, US birth and death records, author calculations. Gestational age in both the Euro-Peristat and US data is based on the best obstetrical estimate available, which in most cases corresponds to first trimester ultrasound. ES, effect size.

Fig 2. State-level FTIMR classification.

The figure shows state level classification: states with good FTIMR (1.25 ≤ FTIMR < 1.75), states with average FTIMR (1.75 ≤ FTIMR < 2.25), states with fair FTIMR (2.25 ≤ FTIMR < 2.75), and states with poor FTIMR (FTIMR ≥ 2.75). All estimates are for full-term infants born in 2010–2012. FTIMR, full-term infant mortality rate.

Fig 3. Group-specific mortality by age of death.

The figure shows the number of infant deaths per 10,000 full-term births in the US by period and overall mortality group (states grouped on the basis of FTIMR: good, 1.25 ≤ FTIMR < 1.75; average, 1.75 ≤ FTIMR < 2.25; fair, 2.25 ≤ FTIMR < 2.75; and poor, FTIMR ≥ 2.75) for the years 2010 to 2012 as well as the percentage of deaths in each age category. Early neonatal mortality is defined as death in the first 6 days after birth. Late neonatal mortality is defined as deaths between 7 and 27 days after birth, and post-neonatal mortality is defined as death 28 to 364 days after birth. FTIMR, full-term infant mortality rate.

Fig 4. Cause-specific mortality rates.

The figure shows the total number of deaths by FTIMR group for the years 2010–2012 as well as the percentage of deaths in each group in the different cause-of-death categories. The following ICD-10 causes of death were included: congenital malformations, Q00–Q99; SUDI, V01–Y89 and R00–R99; perinatal conditions, P00–P96; other, all other ICD-10 codes. Mortality group refers to states grouped on the basis of FTIMR: good (1.25 ≤ FTIMR < 1.75), average (1.75 ≤ FTIMR < 2.25), fair (2.25 ≤ FTIMR < 2.75), and poor (FTIMR ≥ 2.75). FTIMR, full-term infant mortality rate; SUDI, sudden unexpected death in infancy.