Preterm Births — United States, 2006 and 2010

Joyce A. Martin, MPH

Michelle J.K. Osterman, MHS

National Center for Health Statistics, CDC

Corresponding author: Joyce A. Martin, National Center for Health Statistics, CDC. Telephone 301-458-4362; E-mail: JAMartin@cdc.gov.

Introduction

Approximately one third of all infant deaths in the U.S. are related to preterm birth (1). Infants who survive a preterm birth are at greater risk than those born later in pregnancy for early death and lifelong effects such as neurologic and cognitive difficulties (1–4). The rate of preterm births (i.e., <37 completed weeks' gestation) increased approximately 30% during 1981–2006 (5). In 2007, this trend began to reverse; the U.S. preterm birth rate decreased for the fourth consecutive year in 2010, decreasing from the 2006 high of 12.8% to 12.0% in 2010 (5). A total of 4,265,555 births were reported for 2006, including 542,893 preterm births, and 3,999,386 births were reported for 2010, including 478,790 preterm births. Although most of the recent decrease in this rate was among infants born at 34 to 36 weeks' gestation (i.e., late preterm), with a decrease from 9.15% to 8.49% during 2006–2010, the rate of infants born at <34 weeks' gestation (i.e., early preterm) also decreased from 3.66% in 2006 to 3.50% in 2010 (5). Despite improvements in the rate of preterm births, the total number of infants born preterm remains higher than any year during 1981–2001 (5). Substantial differences in preterm birth rates by race/ethnicity persist; additional examination of these differences can provide insight into potential areas for interventions.

The preterm birth analysis and discussion that follows is part of the second CDC Health Disparities and Inequalities Report (CHDIR) and updates information presented in the 2011 CHDIR (6). The 2011 CHDIR (7) was the first CDC report to assess disparities across a wide range of diseases, behavioral risk factors, environmental exposures, social determinants, and health-care access. The topic presented in this report is based on criteria that are described in the 2013 CHDIR Introduction (8). The purposes of this preterm birth report are to raise awareness of racial/ethnic differences among women giving birth to preterm infants and to motivate actions to reduce disparities.

Methods

To assess differences in preterm birth rates by race/ethnicity, CDC analyzed final 2006 and 2010 birth certificate data from the National Vital Statistics System (9). Birth certificates provide demographic and health information on the mother and newborn such as sex, race, ethnicity, gestational age, and geographic region. Geographic region was not analyzed independently because this variable is related to demographic characteristics that can influence preterm birth rates. Comparable information on educational attainment of the mother is not available for the entire national reporting area.

Gestational age measurement is based primarily on the interval between the date of the last normal menses, or last menstrual period (LMP), and the date of birth. The preterm birth rate is defined as births at <37 completed weeks of gestation per 100 total births in a given category; early preterm birth rate is defined as <34 weeks, and late preterm as 34–36 weeks. Race/ethnicity of the mother was self-reported in five categories; white, black, American Indian/Alaska Native (AI/AN), Asian/Pacific Islander (A/PI), and Hispanic. In this report, references to whites, blacks, AI/ANs, and A/PIs refer to non-Hispanic women. Women of Hispanic ethnicity might be of any race or combination of races.

Disparities were measured as the deviations from a referent category rate. Births to non-Hispanic white mothers were used as the referent group for racial/ethnic comparisons. Absolute difference was measured as the simple difference between the rate for a population subgroup and the rate for its respective reference group. The relative difference, a percentage, was calculated by dividing the difference by the value in the referent category and multiplying by 100. The statistical significance of the differences was determined by using the z test at the 95% confidence level (10).

Results

Decreases in preterm births occurred for each of the race/ethnicity groups; white, black, Hispanic, AI/AN, and A/PI from 2006 to 2010 (Table). From 2006 to 2010, the preterm birth rate for black infants decreased by 8% to 17.1%, the lowest level ever reported (5). Despite the decrease, the 2010 preterm rate for black infants (17.1%) was approximately 60% higher than that for white infants (10.8%). AI/AN (13.6%) and Hispanic (11.8%) infants were also at a higher risk for preterm birth in 2010 than white and A/PI infants.

The largest relative differences among the race/ethnicity groups are in early preterm births. Decreases in early preterm births occurred from 2006 to 2010 for white, black, and Hispanic infants. Despite an 8% decrease in the early preterm rate for black infants from 2006 to 2010, the 2010 early preterm birth rate among black infants (6.1%) was double the rate among white (2.9%) and A/PI (2.9%) infants.

The rate of late preterm births declined among each of the race/ethnicity groups during 2006–2010. In 2010, black infants were approximately 40% more likely to be born late preterm than white and A/PI infants. AI/AN and Hispanic infants also were more likely than white and A/PI infants to be born late preterm.

Discussion

Decreases occurred from 2006 to 2010 in preterm birth rates overall and in all racial/ethnic groups examined; however, substantial disparities persisted among racial ethnic groups in 2010. The greatest absolute difference by race/ethnicity in total preterm, early preterm, and late preterm birth rates was among black infants. Black infants have had the highest risk for preterm birth since comparable data on gestational age have been available (1981). The causes of preterm births are not well understood (2). However, disparities among groups might be related to differences in socioeconomic status, prenatal care, maternal risk behaviors, infection, nutrition, preconception stress, and genetics (2).

Limitations

The findings in this report are subject to at least one limitation. The date of the LMP is subject to error from imperfect maternal recall, transcription error, or misidentification of LMP because of postconception bleeding, delayed ovulation, or intervening early miscarriage (5).

Conclusion

Continued reduction in the preterm birth rate is important because approximately one out of every eight infants was born too early in 2010. If the preterm rate continues to decrease at the pace observed from 2006 to 2010, the Healthy People 2020 objective to reduce the rate to 11.4% (objective no. MICH 9-1) (11) will be achieved for the nation overall and for some racial/ethnic groups (i.e., white and A/PI). The 2020 goal for preterm birth rates is further from reach for others; the 2010 rate among blacks (17.1%) must decrease by 50% percent for 2020 (or 5% per year), and the 2010 rate among AI/ANs (13.6%) must decrease by approximately 20% (2% per year). Additional research is needed to clarify the causes of preterm delivery and to develop policies for a future in which preterm birth is a rare event for all populations.

References

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6. Martin JA. Preterm births—United States, 2007. In: CDC health disparities and inequalities report—United States, 2011. MMWR 2011;60(Suppl; January 14, 2011).
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11. US Department of Health and Human Services. Healthy People 2020 topics and objectives. Washington, DC: US Department of Health and Human Services; 2011. Available at http://www.healthypeople.gov/2020/topicsobjectives2020/default.aspx.