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Preterm Births --- United States, 2007

Joyce A. Martin, MPH

Division of Vital Statistics, National Center for Health Statistics, CDC

Corresponding author: Joyce A. Martin, MPH, Division of Vital Statistics, National Center for Health Statistics, CDC, 3311 Toledo Road, Room 7415, Hyattsville, MD 20782. Telephone: 301-458-4362; Fax: 301-458-4033; E-mail: jcm9@cdc.gov.

Preterm infants (those born at <37 completed weeks of gestation) are less likely to survive to their first birthday than infants delivered at higher gestational ages, and those who do survive, especially those born at the earlier end of the preterm spectrum, are more likely to suffer long-term disabilities than infants born at term (1,2). During 1981--2006, the U.S. preterm birth rate increased >30%, from 9.4% to 12.8% of all live births (3). Although lower during 2007 and 2008, the U.S. preterm birth rate remains higher than any year during 1981--2002 (3,4).

Substantial differences in preterm birth rates across racial/ethnic groups have long been noted (5). However, trends in preterm birth rates among the larger racial/ethnic groups often have differed, (3,6). During 1981--2006, rates rose steadily among births to non-Hispanic white mothers, increased modestly among births to Hispanics, and declined slightly for non-Hispanic black births (3). Declines were noted in preterm birth rates in 2007 and 2008 for each of these groups (3,4).

To examine differences in the risk for preterm birth by race/ethnicity, CDC analyzed final 2007 birth certificate data from the National Vital Statistics System (NVSS). For 2007, a total 4,316,211 births were reported, of which 546,602 (12.7%) were preterm (3). For the purposes of this study, gestational age was defined as the interval between the date of the last normal menses and the date of birth; the preterm birth rate is the number of preterm births per 100 total births in a given category. Racial/ethnic origin of the mother are self-reported. National gestational age data according to such attributes as educational attainment, income, and disability status are not available or not collected consistently in NVSS and therefore were not analyzed for this report. Comparable gestational age trend data were not available before the 1981 data year.

In 2007, approximately one of every five infants born to non-Hispanic black mothers was born preterm, compared with one of every eight to nine births to non-Hispanic white and Hispanic women. The 2007 preterm birth rate for non-Hispanic black infants (18.3%) was 59% higher than the rate for non-Hispanic white infants (11.5%) and 49% higher than the rate for Hispanic infants (12.3%) (Table). Among Hispanic groups, 2007 preterm rates ranged from 11.9% of infants born to mothers of Mexican origin to 14.5% of infants born to mothers of Puerto Rican origin.

Rates were higher for infants born to non-Hispanic black mothers at each preterm group: late preterm (34--36 weeks), early preterm (<34 weeks) and extremely preterm (<28 weeks). Non-Hispanic black infants are approximately three times as likely to be delivered extremely preterm as non-Hispanic white and Hispanic infants (1.9% compared with 0.6%). Among the Hispanic groups, extremely preterm birth was most common among births to Puerto Rican mothers (1.0% compared with 0.6%t for all other Hispanic groups).

The findings in this report are subject to at least one limitation. Last normal menses is subject to error from imperfect maternal recall or misidentification because of postconception bleeding, delayed ovulation, or intervening early miscarriage (3).

Although the gap between non-Hispanic whites and non-Hispanic blacks in preterm birth risk has narrowed somewhat during the past two decades, this change is attributable primarily to increases in preterm births among non-Hispanic white infants and not to substantial reductions in short gestations among non-Hispanic blacks (4). Demonstrated causes for the wide disparities in preterm risk by race/ethnicity include differences in socioeconomic status, prenatal care, maternal risk behaviors, infection, nutrition, stress, and genetics (1).

Preterm delivery is a complex problem, and the ability to predict and prevent such births is limited (1). The Healthy People 2010 (HP2010) objective (no. 16-11) to reduce the preterm birth rate for all groups to 7.6% (7) is unlikely to be met; in 2007 the lowest race/ethnicity-specific rate reported, 10.9% for Asian/Pacific Islanders, was >40% higher than the HP2010 target. Expanded research is needed to understand the causes of preterm birth better and to eliminate the wide, persistent racial/ethnic differences in preterm risk.

References

  1. Behrman RE, Butler AS, eds. Preterm birth: causes, consequences, and prevention. Washington, DC: National Academies Press; 2007.
  2. Mathews TJ, MacDorman MF. Infant mortality statistics from the 2006 period linked birth/infant death data set. Hyattsville, MD: US Department of Health and Human Services, CDC, National Center for Health Statistics; 2010. Natl Vital Stat Rep, Vol. 58, No. 17. Available at http://www.cdc.gov/nchs/data/nvsr/nvsr58/nvsr58_17.pdf.
  3. Martin JA, Hamilton BE, Sutton PD, et al. Births: final data for 2007. Hyattsville, MD: US Department of Health and Human Services, CDC, National Center for Health Statistics; 2010. Natl Vital Stat Rep Vol. 58, No. 24.
  4. Martin J, Osterman MJK, Sutton PD. Are preterm births on the decline in the United States? Recent data from the National Vital Statistics System. NCHS Data Brief No. 39. Hyattsville, MD: US Department of Health and Human Services, CDC, National Center for Health Statistics; 2010. Available at http://www.cdc.gov/nchs/data/databriefs/db39.pdf.
  5. CDC. Advance report of final natality statistics, 1981. Monthly Vital Statistics Report. Vl 312. No 9 Supp. DHHS Publ No (PHS)84-1120. Hyattsville, MD: US Department of Health and Human Services, CDC, US Public Health Service, National Center for Health Statistics; 1983.
  6. Brannun Amy, Schoendorf KC. Changing patterns of low birthweight and preterm birth in the United States, 1981--98. Paediatr and Perinat Epidemiol 2002;16:8--15.
  7. US Department of Health and Human Services. Healthy people 2010 midcourse review. 2nd ed. Washington, DC: US Department of Health and Human Services; 2006. Available at http://www.healthypeople.gov.

TABLE. Preterm birth rates*, by race and ethnicity of mother --- National Vital Statistics System, United States, 2007

Race/Ethnicity

Total preterm

Extremely preterm§

Early preterm

Late preterm**

All births

12.7

0.8

3.6

9.0

White, non-Hispanic

11.5

0.6

3.0

8.5

Black, non-Hispanic

18.3

1.9

6.5

11.8

American Indian/Alaska Native

13.9

0.7

4.0

9.9

Asian/Pacific Islander

10.9

0.5

2.8

8.1

All Hispanic

12.3

0.6

3.4

8.9

Mexican origin

11.9

0.6

3.3

8.6

Cuban origin

13.4

0.6

3.4

10.0

Puerto Rican origin

14.5

1.0

4.4

10.0

Central and South American origin

12.1

0.6

3.3

8.8

* Per 100 births in each specified gestational age category.

Birth at <37 weeks' gestation.

§ Birth at <28 weeks' gestation.

Birth at <34 weeks' gestation.

** Birth at 34--36 weeks' gestation.



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