This cross-sectional study examines data from the National Center for Health Statistics for preterm infant births in the United States over a 26-year period to determine trends in infant mortality rates according to race, ethnicity, and socioeconomic status.
Key Points
Question
How have inequalities in US preterm infant mortality changed over time according to a mother’s race and socioeconomic status?
Findings
This cross-sectional study including 12 256 303 preterm infant births over 26 years found widening inequality in preterm infant mortality rates between mothers of differing socioeconomic status, while racial and ethnic disparities remained constant over time. Receiving inadequate antenatal care was the biggest predictor of preterm infant mortality across the study period.
Meaning
These findings indicate that between 1995 and 2020, US preterm infant mortality improved, but racial, ethnic, and socioeconomic inequalities in preterm infant mortality rate persisted.
Abstract
Importance
Inequalities in preterm infant mortality exist between population subgroups within the United States.
Objective
To characterize trends in preterm infant mortality by maternal race and socioeconomic status to assess how inequalities in preterm mortality rates have changed over time.
Design, Setting, and Participants
This was a retrospective longitudinal descriptive study using the US National Center for Health Statistics birth infant/death data set for 12 256 303 preterm infant births over 26 years, between 1995 and 2020. Data were analyzed from December 2022 to March 2023.
Exposures
Maternal characteristics including race, smoking status, educational attainment, antenatal care, and insurance status were used as reported on an infant’s US birth certificate.
Main Outcomes and Measures
Preterm infant mortality rate was calculated for each year from 1995 to 2020 for all subgroups, with a trend regression coefficient calculated to describe the rate of change in preterm mortality.
Results
The average US preterm infant mortality rate (IMR) decreased from 33.71 (95% CI, 33.71 to 34.04) per 1000 preterm births per year between 1995-1997, to 23.32 (95% CI, 23.05 to 23.58) between 2018-2020. Black non-Hispanic infants were more likely to die following preterm births than White non-Hispanic infants (IMR, 31.09; 95% CI, 30.44 to 31.74, vs 21.81; 95% CI, 21.43 to 22.18, in 2018-2020); however, once born, extremely prematurely Black and Hispanic infants had a narrow survival advantage (IMR rate ratio, 0.87; 95% CI, 0.84 to 0.91, in 2018-2020). The rate of decrease in preterm IMR was higher in Black infants (−0.015) than in White (−0.013) and Hispanic infants (−0.010); however, the relative risk of preterm IMR among Black infants compared with White infants remained the same between 1995-1997 vs 2018-2020 (relative risk, 1.40; 95% CI, 1.38 to 1.44, vs 1.43; 95% CI, 1.39 to 1.46). The rate of decrease in preterm IMR was higher in nonsmokers compared with smokers (−0.015 vs −0.010, respectively), in those with high levels of education compared with those with intermediate or low (−0.016 vs – 0.010 or −0.011, respectively), and in those who had received adequate antenatal care compared with those who did not (−0.014 vs −0.012 for intermediate and −0.013 for inadequate antenatal care). Over time, the relative risk of preterm mortality widened within each of these subgroups.
Conclusions and Relevance
This study found that between 1995 and 2020, US preterm infant mortality improved among all categories of prematurity. Inequalities in preterm infant mortality based on maternal race and ethnicity have remained constant while socioeconomic disparities have widened over time.
Introduction
Worldwide, prematurity is the leading cause of infant mortality.1,2 Since the turn of the century, considerable advances in the field of neonatology have resulted in gains in preterm infant survival.3,4,5,6,7 Despite its leading economic status, the United States has one of the top 10 preterm birth rates worldwide,1,8,9 within which there are stark inequalities in the rates of preterm birth and mortality across socioeconomic strata, racial and ethnic groups, and geographic regions.5,10,11,12,13
Preterm-infant mortality rates have been reported on a yearly basis in the United States by the National Center for Health and Statistics (NCHS),5 yet trends in preterm infant mortality have not previously been quantified by race and ethnicity and socioeconomic status (SES) at a national level during the past 2 decades. The objective of this study was to characterize these trends to assess how inequalities in preterm infant mortality rates have changed over time. This is critical for providing public health officials, policymakers, and clinicians with the information necessary to reduce racial and socioeconomic health disparities, as well as determining the effectiveness of existing health policy.
Methods
This retrospective longitudinal cross-sectional descriptive study included 100 million births across the United States over a 26-year period using the period linked birth/infant death data set, which is compiled by the NCHS. Because the national anonymized dataset is publicly available, institutional review board approval was not required or sought for this analysis. We followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guideline.
The period linked birth/infant death data contain all infant birth certificates alongside linked infant birth and death certificates for those who have died in any of the 50 states or District of Columbia in a single year. Between 1995 and 2020, 2 versions of the US standard birth certificates were used, the 1989 certificate and the 2003 revision, which introduced several new data items and was implemented between 2003 and 2016. Within each infant’s birth and death certificate, multiple variables were available for analysis, including maternal and infant characteristics, method of delivery, and underlying cause of death.14 The data are coded uniformly and rigorously quality controlled and, if required, the NCHS will impute missing data. These edited mortality and natality data files are used to create an annual national linked birth-death file for public use. Data from 1995 to 2020 were available at the time of study completion; we performed the analysis from December 2022 to March 2023.
Defining Prematurity
The World Health Organization defines prematurity as any infant born alive before 37 completed weeks of gestation,15 categorizing prematurity as moderate to late preterm (32 to <37 weeks), very preterm (28 to <32 weeks), or extremely preterm (<28 weeks). The Centers for Disease Control and Prevention (CDC) definition of live birth is determined by whether the infant shows signs of life at the point of delivery. This is not limited by duration of pregnancy and is subsequently adapted by individual US states as outlined in the CDC state definitions and reporting requirements.16 To mitigate the effect of nonviable gestations, we chose to only include infants born at 22 weeks or later, categorizing prematurity as follows:
Total preterm (22 to <37 weeks)
Moderately preterm (32 to <37 weeks)
Very preterm (28 to <32 weeks)
Extremely preterm (22 to <28 weeks)
Subgroup Definitions
Subgroups were defined to ensure analogous information was present on both the 1989 and 2003 birth certificates. As maternal race and Hispanic origin were self-reported independently, race and ethnicity data were categorized into Black non-Hispanic (hereafter, Black), Hispanic, and White non-Hispanic (hereafter White). Factors related to maternal SES that were available included maternal education, antenatal care, smoking status, and insurance status.17,18,19 Smoking status was defined by whether a mother had smoked 1 or more cigarettes at some point during the pregnancy. Maternal education was categorized according to the number of years of education completed and grouped into low (high school education or less), intermediate (high school graduate, associated degree, or some college credit), and high (completed a bachelor’s degree or higher). Antenatal care was categorized by the Kessner criteria, determined by the number of antenatal care visits attended relative to the length of gestation.20 Maternal insurance status was categorized as self-funded, private, or government insurance (Medicaid and other government insurance types: federal, state and local); however, insurance data were only available for analyses from 2011 onwards. Single and multiple births were not differentiated so we could evaluate trends across all births.
Subgroup Analyses
The percentage of preterm births in each subgroup was calculated. Infant mortality, defined as a death from 0 to 364 days after birth, was chosen over neonatal mortality to capture all deaths due to prematurity. Mortality rates for each year were calculated stratified by category of prematurity and additionally by individual weeks between 22 and 27 weeks’ gestation, to account for large changes in mortality rate by week at extreme gestations. To evaluate trends in preterm mortality rate, a Poisson regression model was used with year as the single covariate to calculate a regression coefficient reflecting the change in preterm infant mortality rate per year from 1995 to 2020. To counter year-to-year fluctuations, 3-year average mortality rates for 1995-1997 and 2018-2020 were calculated with associated rate ratios to evaluate the change in relative risk of mortality between subgroup categories and a chosen reference over time. Analyses were performed using Stata version 17.
Results
Maternal Characteristics
This study included 103 921 783 infants born across the United States between 1995 and 2020, of whom 12 256 303 were born prematurely, and 437 650 preterm infants died within the first year of life. Between 1995 and 2020, maternal smoking decreased from 13.0% to 6.0%, the percentage of mothers with a bachelor’s degree increased from 22.1% to 33.5%, and receipt of antenatal care remained broadly constant over time. Maternal racial and ethnic composition changed, with a comparable percentage of Black mothers (15.1% vs 16.0%), an increased percentage of Hispanic mothers (16.8% vs 23.8%), and a reduced percentage of White mothers (63.0% vs 59.5%) (Table 1).
Table 1. Change in Percentage Distribution of Preterm Births, 1995-1997 vs 2018-2020.
| Category | 1995-1997 | 2018-2020 | ||
|---|---|---|---|---|
| Births, No. (%) | Preterm births, No. (%) | Births, No. (%) | Preterm births, No. (%) | |
| All infants | 11 682 215 | 1 273 418 (10.9) | 11 178 942 | 1 322 440 (11.6) |
| Race and ethnicitya | ||||
| Black | 1 764 779 (15.1) | 303 081 (17.2) | 1 787 857 (16.0) | 289 196 (16.2) |
| Hispanic | 1 959 323 (16.8) | 208 415 (10.6) | 2 659 183 (23.8) | 318 820 (12.0) |
| White | 7 358 560 (63.0) | 700 952 (9.5) | 6 649 362 (59.5) | 610 170 (9.2) |
| Smoking status | ||||
| Nonsmoker | 8 001 755 (87.0) | 868 970 (10.9) | 10 461 151 (94.0) | 1 202 376 (11.5) |
| Smoker | 1 198 995 (13.0) | 159 690 (13.3) | 667 137 (6.0) | 111 476 (16.7) |
| Education status | ||||
| High | 2 543 658 (22.1) | 223 388 (8.8) | 3 693 586 (33.5) | 347 825 (9.4) |
| Intermediate | 2 542 273 (22.1) | 259 246 (10.2) | 3 093 325 (28.1) | 371 981 (12.0) |
| Low | 6 424 866 (55.8) | 768 903 (12.0) | 4 236 166 (38.4) | 581 079 (13.7) |
| Antenatal care | ||||
| Adequate | 9 366 141 (83.1) | 928 014 (9.9) | 8 821 134 (81.0) | 926 654 (10.5) |
| Intermediate | 1 425 560 (12.7) | 186 180 (13.1) | 1 485 182 (13.6) | 226 087 (15.2) |
| Inadequate | 476 382 (4.2) | 97 605 (20.5) | 589 383 (5.4) | 121 877 (20.7) |
| Insuranceb | (2011-2013) | |||
| Government | 4 746 734 (41.2) | 595 240 (12.5) | 4 905 392 (44.2) | 672 806 (13.7) |
| Private | 4 855 204 (42.1) | 493 820 (10.2) | 5 556 586 (50.0) | 569 381 (10.1) |
| None | 441 342 (3.8) | 47 485 (10.8) | 472 899 (4.3) | 50 118 (10.5) |
As maternal race and Hispanic origin were reported independently, race and ethnicity data were categorized into Black non-Hispanic (Black), Hispanic, and White non-Hispanic (White).
Insurance status was only available on birth certificates from 2011 onwards.
Premature Birth Rates
Between 1995 and 2020, the percentage of premature births (22-37 weeks) gradually oscillated, with a peak of 12.7% in 2006. Consistent with reported patterns, we noted higher rates of preterm birth in mothers who smoked, were less educated, and did not have private insurance and mothers who were Black.21,22,23 Mothers receiving inadequate antenatal care consistently had the highest rate of preterm birth in any subgroup (Table 1).
Trends in Preterm Infant Mortality
During the study period, the total preterm infant mortality rate (IMR) decreased from 33.71 per 1000 preterm births (95% CI, 33.71 to 34.04) in 1995 and 1997, to 23.32 (95% CI, 23.05 to 23.58) in 2018 to 2020. The greatest rate of improvement was seen in very preterm infants (−0.014; 95% CI, −0.016 to −0.013) and extremely preterm infants (−0.014; 95% CI, −0.015 to −0.013), among whom mortality rates increased steeply with decreasing gestational age (Table 2 and eTable 1 in Supplement 1). Improvements in mortality at extreme gestational ages were most apparent from the early 2000s and appear to be continuing. Yet for moderately premature infants, the rate of improvement appeared to slow after 2005 (Figure and eFigure 1 in Supplement 1). Separating each week of extreme gestation, the improvement in preterm mortality rate was smaller for those born at 22 weeks compared with those born between 23 and 27 weeks (Table 3).
Table 2. Change in Risk of Preterm Infant Mortality According to Maternal Characteristics, 1995-1997 vs 2018-2020.
| Category | Prematurity subgroupa | 1995-1997 | 2018-2020 | Regression coefficient (95% CI) | ||
|---|---|---|---|---|---|---|
| Preterm IMRb | Rate ratio (95% CI)c | Preterm IMRb | Rate ratio (95% CI)c | |||
| All infants | Total | 33.71 | 23.32 | −0.014 (−0.014 to −0.014) | ||
| Moderate | 10.51 | 8.30 | −0.008 (−0.009 to −0.007) | |||
| Very | 50.10 | 36.74 | −0.014 (−0.016 to −0.013) | |||
| Extreme | 348.37 | 252.30 | −0.014 (−0.015 to −0.013) | |||
| Race and ethnicityd | ||||||
| Black | Total | 44.51 | 1.40 (1.38 to 1.44) | 31.09 | 1.43 (1.39 to 1.46) | −0.015 (−0.016 to −0.015) |
| Moderate | 11.83 | 1.12 (1.07 to 1.17) | 10.04 | 1.20 (1.14 to 1.26) | −0.008 (−0.009 to −0.006) | |
| Very | 46.74 | 0.87 (0.82 to 0.92) | 38.38 | 0.99 (0.93 to 1.06) | −0.012 (−0.014 to −0.009) | |
| Extreme | 328.02 | 0.88 (0.86 to 0.91) | 238.19 | 0.87 (0.84 to 0.91) | −0.014 (−0.015 to −0.013) | |
| Hispanic | Total | 27.34 | 0.87 (0.84 to 0.89) | 20.13 | 0.92 (0.89 to 0.95) | −0.010 (−0.011 to −0.009) |
| Moderate | 8.68 | 0.82 (0.77 to 0.87) | 7.02 | 0.84 (0.79 to 0.89) | −0.006 (−0.008 to −0.004) | |
| Very | 45.35 | 0.84 (0.78 to 0.91) | 32.20 | 0.83 (0.77 to 0.91) | −0.012 (−0.015 to −0.009) | |
| Extreme | 334.32 | 0.90 (0.86 to 0.95) | 240.09 | 0.88 (0.84 to 0.92) | −0.013 (−0.015 to −0.012) | |
| White | Total | 31.59 | 1 [Reference] | 21.81 | 1 [Reference] | −0.013 (−0.014 to −0.013) |
| Moderate | 10.60 | 8.36 | −0.006 (−0.008 to −0.005) | |||
| Very | 53.72 | 38.65 | −0.016 (−0.017 to −0.014) | |||
| Extreme | 370.67 | 272.43 | −0.014 (−0.015 to −0.013) | |||
| Smoking status | ||||||
| Nonsmoker | Total | 32.83 | 1 [Reference] | 22.50 | 1 [Reference] | −0.015 (−0.015 to −0.014) |
| Moderate | 9.69 | 7.73 | −0.007 (−0.008 to −0.006) | |||
| Very | 48.14 | 35.86 | −0.014 (−0.016 to −0.013) | |||
| Extreme | 348.42 | 251.48 | −0.014 (−0.015 to −0.014) | |||
| Smoker | Total | 39.37 | 1.20 (1.17 to 1.23) | 29.59 | 1.32 (1.27 to 1.36) | −0.010 (−0.011 to −0.009) |
| Moderate | 14.95 | 1.54 (1.47 to 1.62) | 13.87 | 1.79 (1.69 to 1.90) | −0.001 (−0.003 to 0.002) | |
| Very | 57.51 | 1.19 (1.12 to 1.28) | 41.88 | 1.17 (1.07 to 1.28) | −0.014 (−0.017 to −0.011) | |
| Extreme | 333.74 | 0.96 (0.92 to 1.00) | 242.48 | 0.96 (0.91 to 1.02) | −0.012 (−0.014 to −0.010) | |
| Education status | ||||||
| High | Total | 26.89 | 1 [Reference] | 17.91 | 1 [Reference] | −0.016 (−0.017 to −0014) |
| Moderate | 7.78 | 5.64 | −0.010 (−0.012 to −0.008) | |||
| Very | 44.69 | 30.54 | −0.018 (−0.022 to −0.016) | |||
| Extreme | 355.11 | 252.16 | −0.015 (−0.017 to −0014) | |||
| Intermediate | Total | 30.10 | 1.12 (1.08 to 1.16) | 22.49 | 1.26 (1.21 to 1.30) | −0.010 (−0.011 to −0.009) |
| Moderate | 8.70 | 1.12 (1.05 to 1.20) | 7.82 | 1.39 (1.30 to 1.48) | −0.001 (−0.003 to 0.001) | |
| Very | 44.55 | 1.00 (0.91 to 1.09) | 35.58 | 1.17 (1.07 to 1.27) | −0.012 (−0.014 to −0.010) | |
| Extreme | 330.44 | 0.93 (0.88 to 0.98) | 238.42 | 0.95 (0.90 to 0.99) | −0.015 (−0.016 to −0.013) | |
| Low | Total | 33.78 | 1.26 (1.22 to 1.29) | 25.27 | 1.41 (1.37 to 1.45) | −0.011 (−0.011 to −0.010) |
| Moderate | 11.29 | 1.45 (1.38 to 1.53) | 9.83 | 1.74 (1.66 to 1.83) | −0.004 (−0.005 to −0.003) | |
| Very | 49.82 | 1.11 (1.04 to 1.19) | 38.15 | 1.25 (1.17 to 1.34) | −0.010 (−0.012 to −0.009) | |
| Extreme | 325.14 | 0.92 (0.88 to 0.95) | 244.12 | 0.97 (0.93 to 1.01) | −0.012 (−0.012 to −0.011) | |
| Antenatal care | ||||||
| Adequate | Total | 29.24 | 1 [Reference] | 19.96 | 1 [Reference] | −0.014 (−0.015 to −0.014) |
| Moderate | 9.03 | 6.70 | −0.010 (−0.011 to −0.009) | |||
| Very | 43.35 | 30.73 | −0.016 (−0.017 to −0.014) | |||
| Extreme | 324.16 | 236.67 | −0.014 (−0.015 to −0.013) | |||
| Intermediate | Total | 33.91 | 1.16 (1.13 to 1.19) | 24.85 | 1.25 (1.21 to 1.28) | −0.012 (−0.013 to −0.011) |
| Moderate | 11.74 | 1.30 (1.24 to 1.37) | 9.90 | 1.48 (1.40 to 1.55) | −0.005 (−0.007 to −0.003) | |
| Very | 64.48 | 1.49 (1.39 to 1.59) | 48.80 | 1.59 (1.47 to 1.71) | −0.012 (−0.015 to −0.010) | |
| Extreme | 350.05 | 1.08 (1.03 to 1.13) | 259.34 | 1.10 (1.05 to 1.15) | −0.012 (−0.014 to −0.011) | |
| Inadequate | Total | 46.30 | 1.58 (1.52 to 1.65) | 33.95 | 1.70 (1.63 to 1.77) | −0.013 (−0.014 to −0.011) |
| Moderate | 16.14 | 1.79 (1.66 to 1.92) | 14.48 | 2.16 (2.02 to 2.31) | −0.005 (−0.007 to −0.002) | |
| Very | 62.88 | 1.45 (1.31 to 1.60) | 54.83 | 1.78 (1.61 to 1.98) | −0.007 (−0.011 to −0.004) | |
| Extreme | 356.41 | 1.10 (1.04 to 1.17) | 283.28 | 1.20 (1.12 to 1.28) | −0.009 (−0.010 to −0.007) | |
Abbreviations: IMR, infant mortality rate; RR, rate ratio.
Prematurity categories were defined as follows: total (22 to <37 weeks), moderate (32 to <37 weeks), very (28 to <32 weeks), extreme (22 to <28 weeks).
Deaths per 1000 preterm births.
Comparing maternal categories to designated reference within each prematurity subgroup.
As maternal race and Hispanic origin were reported independently, race and ethnicity data were categorized into Black non-Hispanic (Black), Hispanic, and White non-Hispanic (White).
Figure. Trends in Preterm Infant Mortality Rate (IMR) Between 1995 and 2020 According to Maternal Race and Ethnicity.
Trends for all premature infants (gestation of 22 to <37 weeks) and moderately (32 to <37 weeks), very (28 to <32 weeks), and extremely (22 to <28 weeks) premature infants. Axis scaled for each category of prematurity. Shading indicates 95% CIs.
Table 3. Change in Risk of Extreme Preterm Infant Mortality According to Maternal Characteristics, 1995-1997 to 2018-2020.
| Category | Week of gestation | 1995-1997 | 2018-2020 | Regression coefficient (95% CI) | ||
|---|---|---|---|---|---|---|
| Preterm IMRa | Rate ratio (95% CI)b | Preterm IMRa | Rate ratio (95% CI)b | |||
| All infants | 22 | 801.96 | 727.90 | −0.003 (−0.004 to −0.002) | ||
| 23 | 670.71 | 491.62 | −0.012 (−0.013 to −0.011) | |||
| 24 | 449.05 | 305.94 | −0.017 (−0.019 to −0.016) | |||
| 25 | 286.24 | 199.00 | −0.016 (−0.018 to −0.015) | |||
| 26 | 197.13 | 131.96 | −0.018 (−0.020 to −0.016) | |||
| 27 | 132.00 | 96.75 | −0.016 (−0.018 to −0.014) | |||
| Race and ethnicityc | ||||||
| Black | 22 | 763.57 | 0.90 (0.83 to 0.97) | 690.40 | 0.90 (0.81 to 0.99) | −0.004 (−0.006 to −0.001) |
| 23 | 618.37 | 0.85 (0.79 to 0.91) | 443.22 | 0.79 (0.72 to 0.87) | −0.013 (−0.015 to −0.011) | |
| 24 | 382.98 | 0.76 (0.70 to 0.82) | 257.52 | 0.72 (0.65 to 0.79) | −0.017 (−0.020 to −0.015) | |
| 25 | 249.25 | 0.78 (0.72 to 0.84) | 190.24 | 0.86 (0.78 to 0.95) | −0.014 (−0.016 to −0.011) | |
| 26 | 180.34 | 0.85 (0.78 to 0.92) | 120.18 | 0.78 (0.69 to 0.87) | −0.019 (−0.022 to −0.016) | |
| 27 | 124.65 | 0.90 (0.81 to 0.99) | 86.79 | 0.84 (0.74 to 0.95) | −0.016 (−0.019 to −0.012) | |
| Hispanic | 22 | 765.53 | 0.90 (0.80 to 1.01) | 711.49 | 0.93 (0.83 to 0.99) | −0.001 (−0.004 to 0.002) |
| 23 | 640.47 | 0.88 (0.79 to 0.98) | 467.68 | 0.84 (0.75 to 0.93) | −0.012 (−0.015 to −0.009) | |
| 24 | 448.50 | 0.89 (0.80 to 0.99) | 292.04 | 0.82 (0.73 to 0.91) | −0.019 (−0.022 to −0.015) | |
| 25 | 279.98 | 0.87 (0.78 to 0.98) | 177.66 | 0.80 (0.71 to 0.91) | −0.019 (−0.022 to −0.015) | |
| 26 | 189.37 | 0.89 (0.78 to 1.01) | 113.34 | 0.73 (0.64 to 0.85) | −0.020 (−0.025 to −0.016) | |
| 27 | 127.44 | 0.92 (0.80 to 1.06) | 98.69 | 0.95 (0.82 to 1.10) | −0.016 (−0.021 to −0.012) | |
| White | 22 | 848.70 | 1 [Reference] | 768.19 | 1 [Reference] | −0.003 (−0.005 to −0.001) |
| 23 | 728.54 | 558.72 | −0.011 (−0.013 to −0.009) | |||
| 24 | 504.72 | 357.53 | −0.017 (−0.019 to −0.015) | |||
| 25 | 320.74 | 221.17 | −0.016 (−0.019 to −0.014) | |||
| 26 | 212.89 | 154.31 | −0.016 (−0.018 to −0.013) | |||
| 27 | 138.74 | 103.93 | −0.016 (−0.019 to −0.013) | |||
| Smoking status | ||||||
| Nonsmoker | 22 | 803.15 | 1 [Reference] | 731.37 | 1 [Reference] | −0.002 (−0.006 to 0.002) |
| 23 | 674.35 | 487.18 | −0.007 (−0.011 to −0.003) | |||
| 24 | 446.46 | 300.84 | −0.015 (−0.019 to −0.011) | |||
| 25 | 284.79 | 199.46 | −0.018 (−0.023 to −0.013) | |||
| 26 | 191.61 | 130.95 | −0.015 (−0.020 to −0.010) | |||
| 27 | 133.28 | 96.89 | −0.013 (−0.018 to −0.007) | |||
| Smoker | 22 | 776.53 | 0.97 (0.87 to 1.08) | 662.62 | 0.91 (0.78 to 0.99) | −0.003 (−0.004 to −0.001) |
| 23 | 623.82 | 0.93 (0.83 to 1.03) | 510.30 | 1.05 (0.91 to 1.20) | −0.013 (−0.014 to −0.011) | |
| 24 | 430.32 | 0.96 (0.87 to 1.07) | 338.44 | 1.12 (0.98 to 1.29) | −0.018 (−0.019 to −0.016) | |
| 25 | 296.27 | 1.04 (0.93 to 1.16) | 184.43 | 0.92 (0.79 to 1.08) | −0.016 (−0.018 to −0.014) | |
| 26 | 213.14 | 1.11 (1.00 to 1.24) | 137.52 | 1.05 (0.89 to 1.23) | −0.019 (−0.021 to −0.017) | |
| 27 | 129.77 | 0.97 (0.86 to 1.11) | 88.84 | 0.92 (0.76 to 1.10) | −0.018 (−0.020 to −0.015) | |
| Education status | ||||||
| High | 22 | 817.35 | 1 [Reference] | 752.91 | 1 [Reference] | −0.003 (−0.006 to 0.000) |
| 23 | 719.20 | 524.90 | −0.014 (−0.016 to −0.011) | |||
| 24 | 485.94 | 316.40 | −0.019 (−0.022 to −0.016) | |||
| 25 | 289.14 | 164.16 | −0.021 (−0.025 to −0.017) | |||
| 26 | 192.73 | 114.51 | −0.022 (−0.026 to −0.018) | |||
| 27 | 120.54 | 96.41 | −0.014 (−0.019 to −0.010) | |||
| Intermediate | 22 | 789.18 | 0.97 (0.85 to 1.10) | 717.12 | 0.95 (0.85 to 1.10) | −0.004 (−0.007 to −0.001) |
| 23 | 635.47 | 0.88 (0.79 to 0.99) | 451.13 | 0.86 (0.79 to 0.99) | −0.014 (−0.017 to −0.012) | |
| 24 | 443.56 | 0.91 (0.81 to 1.03) | 287.06 | 0.91 (0.81 to 1.03) | −0.019 (−0.022 to −0.016) | |
| 25 | 266.31 | 0.92 (0.81 to 1.05) | 188.62 | 1.15 (0.81 to 1.05) | −0.013 (−0.017 to −0.009) | |
| 26 | 135.49 | 0.70 (0.61 to 0.81) | 122.80 | 1.07 (0.61 to 0.81) | −0.013 (−0.017 to −0.009) | |
| 27 | 124.01 | 1.03 (0.88 to 1.20) | 94.87 | 0.98 (0.88 to 1.20) | −0.018 (−0.022 to −0.014) | |
| Low | 22 | 758.92 | 0.93 (0.83 to 1.04) | 697.07 | 0.93 (0.83 to 1.04) | −0.002 (−0.004 to 0.000) |
| 23 | 628.63 | 0.87 (0.79 to 0.96) | 476.36 | 0.91 (0.79 to 0.96) | −0.009 (−0.011 to −0.008) | |
| 24 | 413.81 | 0.85 (0.77 to 0.94) | 295.79 | 0.93 (0.77 to 0.94) | −0.014 (−0.016 to −0.012) | |
| 25 | 274.46 | 0.95 (0.85 to 1.06) | 200.21 | 1.22 (0.85 to 1.06) | −0.014 (−0.016 to −0.011) | |
| 26 | 190.28 | 0.99 (0.88 to 1.11) | 126.68 | 1.11 (0.88 to 1.11) | −0.015 (−0.018 to −0.013) | |
| 27 | 129.37 | 1.07 (0.94 to 1.22) | 92.63 | 0.96 (0.94 to 1.22) | −0.013 (−0.015 to −0.010) | |
| Antenatal care | ||||||
| Adequate | 22 | 820.64 | 1 [Reference] | 713.68 | 1 [Reference] | −0.003 (−0.005 to −0.001) |
| 23 | 673.30 | 480.67 | −0.012 (−0.013 to −0.010) | |||
| 24 | 445.51 | 293.78 | −0.017 (−0.019 to −0.015) | |||
| 25 | 282.15 | 188.24 | −0.016 (−0.017 to −0.014) | |||
| 26 | 186.58 | 118.53 | −0.018 (−0.020 to −0.016) | |||
| 27 | 122.79 | 91.94 | −0.016 (−0.018 to −0.013) | |||
| Intermediate | 22 | 866.82 | 1.01 (0.91 to 1.12) | 754.84 | 1.06 (0.95 to 1.18) | −0.001 (−0.004 to 0.003) |
| 23 | 745.30 | 1.05 (0.95 to 1.17) | 495.76 | 1.03 (0.92 to 1.15) | −0.011 (−0.014 to −0.008) | |
| 24 | 505.17 | 1.07 (0.96 to 1.20) | 335.87 | 1.14 (1.02 to 1.29) | −0.015 (−0.018 to −0.011) | |
| 25 | 329.46 | 1.09 (0.96 to 1.24) | 207.43 | 1.10 (0.96 to 1.26) | −0.017 (−0.021 to −0.013) | |
| 26 | 235.05 | 1.21 (1.09 to 1.35) | 145.54 | 1.23 (1.09 to 1.39) | −0.016 (−0.019 to −0.012) | |
| 27 | 177.85 | 1.38 (1.23 to 1.56) | 105.23 | 1.14 (1.00 to 1.31) | −0.016 (−0.021 to −0.012) | |
| Inadequate | 22 | 784.63 | 0.91 (0.79 to 1.04) | 675.64 | 0.95 (0.82 to 1.10) | 0.000 (−0.004 to 0.003) |
| 23 | 659.90 | 0.93 (0.80 to 1.06) | 481.03 | 1.00 (0.86 to 1.16) | −0.007 (−0.010 to −0.003) | |
| 24 | 466.63 | 0.99 (0.85 to 1.14) | 316.81 | 1.08 (0.92 to 1.27) | −0.014 (−0.018 to −0.010) | |
| 25 | 315.54 | 1.05 (0.89 to 1.23) | 217.36 | 1.15 (0.97 to 1.38) | −0.010 (−0.015 to −0.006) | |
| 26 | 252.91 | 1.27 (1.10 to 1.48) | 163.31 | 1.38 (1.16 to 1.64) | −0.012 (−0.017 to 0.007) | |
| 27 | 159.90 | 1.20 (1.00 to 1.44) | 107.91 | 1.17 (0.96 to 1.43) | −0.013 (−0.018 to −0.007) | |
Abbreviations: IMR, infant mortality rate; RR, rate ratio.
Deaths per 1000 preterm births.
Comparing maternal categories to designated reference.
As maternal race and Hispanic origin were reported independently, race and ethnicity data were categorized into Black non-Hispanic (Black), Hispanic, and White non-Hispanic (White).
Trends in Preterm Infant Mortality by Maternal Race
Across the study period, Black infants were more likely to die after preterm birth than Hispanic and White infants; however, once born, extremely premature Black and Hispanic infants had a narrow survival advantage over White infants (Table 2 and Table 3). Comparing 1995-1997 and 2018-2020, the total preterm IMR decreased from 44.51 per 1000 preterm births (95% CI, 43.74 to 45.27) to 31.09 (95% CI, 30.44 to 31.74) for Black infants, 27.34 per 1000 (95% CI, 26.62 to 28.06) to 20.13 (95% CI, 19.63 to 20.62) for Hispanic infants, and 31.59 per 1000 (95% CI, 31.16 to 32.01) to 21.81 (95% CI, 21.43 to 22.18) for White infants. The rate of decrease in total preterm IMR was higher in Black infants (−0.015; 95% CI, −0.016 to −0.015) than in Hispanic infants (−0.0010; 95%CI, −0.011 to −0.009) and White infants (−0.013; 95% CI, −0.014 to −0.013); however, the relative risk of preterm IMR among Black and Hispanic infants compared with White infants remained the constant (relative risk, 1.40; 95% CI, 1.38 to 1.44, vs 1.43; 95% CI, 1.39 to 1.46) (Table 2 and eTable 1 in Supplement 1). This trend was seen across all categories of prematurity (Table 2). For infants born extremely prematurely (22-27 weeks), both mortality rates and the rate of improvement were comparable between racial and ethnic groups, and there were no changes in the relative risks of preterm infant mortality among Black and Hispanic infants compared with White infants (Table 3).
Trends in Preterm Infant Mortality by Factors Related to Maternal SES
Across all categories of prematurity, mothers receiving inadequate antenatal care had the highest preterm mortality rate, with mothers who smoked, who were less educated, or who lacked private insurance having higher preterm mortality rates within their respective subgroups (Table 2). For infants born extremely prematurely, mortality rates were comparable within subgroups, except for lower mortality rates in infants who received adequate antenatal care (Table 2 and Table 3). Over time, the rate of decrease in total preterm IMR was higher in nonsmokers compared with smokers (−0.015 vs −0.010, respectively), higher in those with high levels of education compared with those with intermediate or low (−0.016 vs – 0.010 or −0.011, respectively), and higher in those who had received adequate vs intermediate or inadequate antenatal care (−0.014 vs −0.012 and −0.013, respectively). Correspondingly, the relative risk of total preterm infant mortality widened within these subgroups (Table 2). Once split into each week of extreme prematurity (22-27 weeks), the rates of improvement in preterm IMR were comparable between subgroups, and there were no differences in the relative risk of preterm infant mortality (Table 3 and eTable 2 in Supplement 1). Full trends in mortality rates in 1995 to 2020 are displayed in eFigures 2-4 in Supplement 1.
Insurance status was only available for analysis in data from 2011 to 2018. Over time, the relative risk of preterm mortality between government and private insurance remained constant (relative risk, 0.84; 95% CI, 0.81-0.86, vs 0.82; 95% CI, 0.80-0.84) while there was a suggestion of narrowing between those mothers without insurance (relative risk, 1.21; 95% CI, 1.16-1.26, vs 1.12; 95% CI, 1.06-1.17) (Table 4 and eTable 3 in Supplement 1). For infants born extremely prematurely, preterm mortality rates were similar between insurance groups, apart from the group at 22 to 23 weeks’ gestation, where mothers with government insurance fared better. Survival gains at extreme prematurity were equitable across insurance groups.
Table 4. Change in Risk of Preterm Infant Mortality According to Maternal Insurance Status and Prematurity Category or Week of Gestation, 2011-2013 vs 2018-2020.
| Insurance type | Prematurity subgroup or week of gestationa | 2011-2013 | 2018-2020 | Regression coefficient (95% CI) | ||
|---|---|---|---|---|---|---|
| Preterm IMRb | Rate ratio (95% CI)c | Preterm IMRb | Rate ratio (95% CI)c | |||
| By prematurity subgroup | ||||||
| Government | Total | 28.07 | 1 [Reference] | 25.03 | 1 [Reference] | −0.016 (−0.019 to −0.013) |
| Moderate | 9.78 | 9.64 | −0.002 (−0.008 to −0.003) | |||
| Very | 39.43 | 38.51 | −0.005 (−0.013 to −0.002) | |||
| Extreme | 276.62 | 243.33 | −0.018 (−0.021 to −0.014) | |||
| Private | Total | 23.47 | 0.84 (0.81 to 0.86) | 20.53 | 0.82 (0.80 to 0,84) | −0.020 (−0.023 to −0.017) |
| Moderate | 6.65 | 0.68 (0.63 to 0.73) | 6.55 | 0.68 (0.64 to 0.72) | −0.003 (−0.010 to 0.004) | |
| Very | 34.35 | 0.87 (0.81 to 0.93) | 33.65 | 0.87 (0.81 to 0.93) | −0.007 (−0.016 to 0.002) | |
| Extreme | 293.68 | 1.06 (1.03 to 1.10) | 258.02 | 1.06 (1.02 to 1.10) | −0.019 (−0.024 to −0.015) | |
| None | Total | 33.93 | 1.21 (1.16 to 1.26) | 27.92 | 1.12 (1.06 to 1.17) | −0.029 (−0.039 to −0.019) |
| Moderate | 10.44 | 1.07 (0.97 to 1.17) | 9.35 | 0.97 (0.87 to 1.07) | −0.017 (−0.036 to 0.002) | |
| Very | 42.42 | 1.08 (0.93 to 1.22) | 39.20 | 1.02 (0.87 to 1.16) | −0.007 (−0.033 to 0.018) | |
| Extreme | 325.18 | 1.18 (1.10 to 1.25) | 286.70 | 1.18 (1.10 to 1.26) | −0.020 (−0.032 to −0.007) | |
| By gestation week | ||||||
| Government | Week 22 | 733.73 | 1 [Reference] | 697.52 | 1 [Reference] | −0.007 (−0.015 to 0.001) |
| Week 23 | 564.46 | 471.80 | −0.027 (−0.035 to −0.019) | |||
| Week 24 | 338.80 | 288.31 | −0.023 (−0.033 to −0.014) | |||
| Week 25 | 215.75 | 199.40 | −0.008 (−0.018 to 0.002) | |||
| Week 26 | 151.69 | 131.84 | −0.019 (−0.030 to −0.007) | |||
| Week 27 | 102.84 | 92.93 | −0.012 (−0.025 to 0.001) | |||
| Private | Week 22 | 829.97 | 1.13 (1.04 to 1.23) | 760.14 | 1.09 (0.99 to 1.19) | −0.013 (−0.022 to −0.004) |
| Week 23 | 597.14 | 1.06 (0.89 to 1.23) | 514.32 | 1.09 (0.90 to 1.28) | −0.022 (−0.031 to −0.012) | |
| Week 24 | 360.54 | 1.06 (0.89 to 1.24) | 321.34 | 1.11 (0.93 to 1.30) | −0.018 (−0.029 to −0.008) | |
| Week 25 | 233.34 | 1.08 (0.90 to 1.27) | 195.38 | 0.98 (0.79 to 1.17) | −0.023 (−0.035 to −0.011) | |
| Week 26 | 133.80 | 0.88 (0.68 to 1.08) | 133.55 | 1.01 (0.79 to 1.24) | −0.005 (−0.019 to 0.009) | |
| Week 27 | 98.03 | 0.95 (0.72 to 1.19) | 101.43 | 1.09 (0.83 to 1.35) | 0.005 (−0.011 to 0.020) | |
| None | Week 22 | 753.43 | 1.03 (0.86 to 1.20) | 771.92 | 1.11 (0.92 to 1.29) | 0.003 (−0.022 to 0.027) |
| Week 23 | 596.00 | 1.06 (0.88 to 1.23) | 512.50 | 1.09 (0.91 to 1.27) | −0.020 (−0.046 to 0.006) | |
| Week 24 | 393.31 | 1.16 (0.98 to 1.34) | 338.49 | 1.17 (0.98 to 1.26) | −0.026 (−0.056 to 0.003) | |
| Week 25 | 252.69 | 1.17 (0.98 to 1.36) | 204.95 | 1.03 (0.81 to 1.25) | −0.027 (−0.061 to 0.007) | |
| Week 26 | 153.39 | 1.01 (0.78 to 1.24) | 125.38 | 0.95 (0.70 to 1.21) | −0.036 (−0.078 to 0.006) | |
| Week 27 | 125.26 | 1.22 (0.98 to 1.45) | 93.05 | 1.00 (0.72 to 1.29) | −0.053 (−0.099 to −0.007) | |
Abbreviations: IMR, infant mortality rate; RR, rate ratio.
Prematurity categories were defined as follows: total (22 to <37 weeks), moderate (32 to <37 weeks), very (28 to <32 weeks), extreme (22 to <28 weeks).
Deaths per 1000 preterm births.
Comparing maternal categories to designated reference.
Discussion
To our knowledge, this is the first population study to explore trends in preterm infant mortality in the United States over 2 decades according to maternal race and ethnicity and factors associated with maternal SES. Robust national birth statistics reporting by the NCHS allowed us to include more than 100 million births over a 26-year period. Our study shows that in the United States, across all categories of prematurity, there has been significant improvement in preterm infant mortality. Yet the rates of improvement have differed between population subgroups, with social, racial, and ethnic disparities in preterm mortality remaining present.
The improvement in preterm infant mortality rates in the United States between 1995 and 2020 mirrors trends seen globally in high-income settings4 and reflects known improvements in multiple facets of obstetric and neonatal care. This includes the use and timing of antenatal steroids, exogenous surfactant, and other facets of respiratory care as well as antibiotics, neonatal thermoregulation, and nutrition.3,24,25,26,27,28,29 Other studies have shown these interventions have reduced neonatal morbidity and improved neurodevelopmental outcomes.24,30 These clinical and technological advances have been focused on infants born at the extremes of gestation, where mortality rate is highest. We correspondingly found the greatest survival gains in very and extremely premature infants, most notably in those born between 23 and 27 weeks’ gestation. Smaller survival gains for infants born at 22 weeks could be attributed to the severity of causative pathology, the proximity to the limit of viability, and variation in decision-making around the initiation and continuation of survival focused care, and therefore, these findings should be interpreted with caution.
Overall, the improvements in mortality at extreme gestational ages were most apparent from the early 2000s and appear to be continuing. In contrast, for moderately premature infants, the rate of improvement appears to slow after 2005 (Figure). This might suggest that we are close to reaching the natural limit in mortality reduction among moderately premature infants, but opportunity remains to reduce mortality rates among extremely premature infants. It will be instructive to see how this develops in subsequent years, particularly with increasing numbers of infants born at 22 weeks’ gestation receiving active survival-focused care.
Reducing racial inequalities in preterm infant mortality is a major public health problem in the United States. It is already known that Black infants are 1.5 to 2 times more likely to be born prematurely than White infants, with higher proportions of multiple births among Black infants.5,6,31,32,33,34 The cause of these differences is believed to be multifactorial; however, the relative contribution of different factors is debated.21,31,35,36,37,38 Although studies of population-based data sets have looked at racial disparities in preterm mortality and morbidity, this is the first study to our knowledge to quantify this over 2 decades at a national level in the United States.39,40
The finding that Black infants remain approximately 1.4 times more likely to die from prematurity than White infants is concerning. It is thought this difference is partially attributed to a higher proportion of Black infants being born at the extremes of prematurity.41 Indeed, among infants born extremely prematurely, we found mortality rates were comparable between racial and ethnic groups, with a slight survival advantage for Black and Hispanic infants compared with White infants. This may reflect pathophysiological differences in the cause of extreme prematurity between racial groups, and it has been suggested that the balance between maternal vs infant ill health resulting in extreme prematurity may differ.40 This is yet to be characterized.
For infants born moderately, very, or extremely prematurely, survival gains were similar between all racial and ethnic groups over the study period. This may suggest that at a national level, once infants are born prematurely, the neonatal care received by different racial and ethnic groups is broadly equitable, with all groups having access to the advancements in neonatal care accounting for the majority of the improvement in preterm infant survival. Regional and neonatal center–specific racial disparities have been reported, and therefore work is needed to characterize trends at a local level.42,43,44
Regarding socioeconomic factors, our findings are consistent with previously reported disparities in both preterm birth rate and infant mortality in relation to mothers’ education, insurance, antenatal care, and smoking status.45,46,47 Antenatal care status was the biggest predictor of both preterm birth and preterm infant mortality and was likely the strongest marker of deprivation within our analyses, with complex financial, structural, and geographical barriers all contributing to mothers’ ability to access adequate antenatal care.48,49 Mothers unable to access antenatal care would not have benefited from advances in antenatal care practice, which may partly account for the widening inequality in preterm mortality seen. This adds to an economic case for free antenatal care for all pregnant individuals in the United States.50,51,52 Although we note increasing levels of maternal education, the widening inequality in preterm infant mortality among mothers with differing educational attainment confirms the relationship between education status and maternal and child health.53 Insurance status should be interpreted in the context of the shortened analysis time frame for those data (2011-2020). The fact mothers without insurance consistently had the highest mortality rates may reflect their underlying SES. Ensuring all mothers have appropriate insurance is important; however, previous studies have debated the effect of the expansion of Medicaid on infant mortality.54,55
As seen with maternal race, among infants born extremely prematurely, equitable reductions in mortality rate were seen among subgroups. This again suggests that at a national level, most infants receive equitable neonatal care once born. Further analysis of the etiology resulting in extreme prematurity within subgroups would aid interpretation of this finding.30,31,32 This highlights the need for targeted health care and economic policy focused on preventing extreme prematurity, as a strategy to reduce social, racial, and ethnic disparities in preterm health. The gestational age at which an infant is born remains the major determinant of their risk of death.
Strengths and Limitations
The key strengths of our study include the size and validity of the NCHS period linked birth/infant death data set, the length of time studied, and the variety of maternal characteristics assessed. Our study has several limitations. First, we were unable to calculate adjusted odds ratios to account for confounders because of the separation of the data files containing the denominator data (live births) and numerator data (infant deaths). This meant our analyses were limited to reporting overall unadjusted trends and could not explore confounding or test for interaction. Second, despite the CDC definition of live birth remaining consistent across the study period, we were unable to account for inconsistencies in the interpretation of live birth between US states.16,56,57 We sought to mitigate this by using a 22-week gestational age cutoff as well as reporting national trends. Beginning with 2014, the NCHS transitioned to using the obstetric estimate for estimating gestational age, replacing last normal menses. This could not be adjusted for without shortening the analysis time frame. Lastly, other metrics of SES, such as occupation and income, as well as indexes of deprivation, such as the Multidimensional Deprivation Index, were not available in the data set, narrowing the factors known to be associated with SES that were available for analysis.58 Further analyses of maternal comorbidities such as diabetes, hypertension, and obesity could have been instructive.59,60,61
Conclusions
This population-level study analyzing trends in US preterm mortality demonstrated a steady improvement in preterm mortality rate across all categories of prematurity. Yet racial, ethnic, and socioeconomic disparities persist. Our findings suggest that the proportional inequalities in total preterm IMR between Black and other racial and ethnic groups have remained constant over time. Unfortunately, we found an overall widening of the inequality in preterm mortality among mothers from socioeconomically disadvantaged groups. Our results confirm the critical importance of mothers receiving antenatal care.
eTable 1. The change in risk of preterm infant mortality according to maternal characteristics, 1995-1997 vs 2018-2020
eTable 2. The change in risk of extreme preterm infant mortality according to maternal characteristics, 1995-1997 to 2018-2020
eTable 3. The change in risk of preterm infant mortality for according to maternal insurance status, 2011-2013 vs 2018-2020
eFigure 1. Trends in preterm infant mortality rate between 1995 and 2020 for all US preterm infants
eFigure 2. Trends in preterm infant mortality rate between 1995 and 2020 according to maternal education status
eFigure 3. Trends in preterm infant mortality rate between 1995 and 2020 according to maternal antenatal care status
eFigure 4. Trends in preterm infant mortality rate between 1995 and 2020 according to maternal smoking status
Data sharing statement
References
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
eTable 1. The change in risk of preterm infant mortality according to maternal characteristics, 1995-1997 vs 2018-2020
eTable 2. The change in risk of extreme preterm infant mortality according to maternal characteristics, 1995-1997 to 2018-2020
eTable 3. The change in risk of preterm infant mortality for according to maternal insurance status, 2011-2013 vs 2018-2020
eFigure 1. Trends in preterm infant mortality rate between 1995 and 2020 for all US preterm infants
eFigure 2. Trends in preterm infant mortality rate between 1995 and 2020 according to maternal education status
eFigure 3. Trends in preterm infant mortality rate between 1995 and 2020 according to maternal antenatal care status
eFigure 4. Trends in preterm infant mortality rate between 1995 and 2020 according to maternal smoking status
Data sharing statement
