Preterm Singleton Births -- United States, 1989-1996

Preterm birth (birth at less than 37 completed weeks of gestation) is the second leading cause of neonatal mortality in the United States (1). Preterm birthrates differ by race; in 1996, black infants were 1.8 times more likely than white infants to be preterm (2). From 1989 through 1996, the overall rate of preterm birth (per 1000 live-born infants) increased 4% (2), and the rate of multiple births (e.g., twins, triplets, or other higher-order births) increased 19% (2). Multiple births are associated with preterm birth (3); trends in preterm births independent of the influence of multiple births have not been fully explored. To characterize race- and ethnicity-specific trends in preterm birth independent of multiple births, data from U.S. birth certificates for 1989-1996 were analyzed for singleton births only. This report summarizes the results of this analysis and indicates that although singleton preterm birthrates are stable overall, substantial changes in rates occurred in some racial/ethnic subgroups.

For this report, preterm birth was defined as a live birth occurring at 17-36 completed weeks of gestation and was subgrouped by weeks of gestation: moderately preterm (33-36 weeks), very preterm (29-32 weeks), extremely preterm (20-28 weeks), and ultra preterm (17-19 weeks). Gestational age was determined from information on the birth certificate by one of two methods (2,4): 1) the interval between the first day of the mother's last normal menstrual period (LMP) and the date of birth, or 2) a clinical estimate by the birth attendant of gestational age when the month or year of the LMP was missing or when the gestational age based on this date was inconsistent with the infant's birth weight. Approximately 1% of singleton infants were excluded because of missing or implausible estimates of gestational age. Infants were imputed as singletons for the 0.02% of live-born infants for which the number of fetuses in a given pregnancy was unreported. Maternal race and ethnicity were based on self-report and categorized as non-Hispanic white, non-Hispanic black, Hispanic, American Indian/Alaskan Native, or Asian/Pacific Islander. Stratification by gestational age was not performed for American Indians/Alaskan Natives and Asians/Pacific Islanders because the number of preterm births, when broken down into gestational age subgroups, was too small for meaningful analysis.

From 1989 through 1996, the preterm birthrate (per 1000 live-born infants) among singletons increased 0.3% (from 97.0 to 97.3) (Figure_1). Among moderately preterm singleton infants, the birthrate increased 2% (from 74.8 to 76.5). Among very preterm singleton infants, the birthrate decreased 8% (from 14.4 to 13.2) and among extremely preterm infants, decreased 4% (from 7.6 to 7.3) (Table_1). The singleton preterm birthrate increased 8% among non-Hispanic whites but decreased 10% among non-Hispanic blacks, 4% among Hispanics, 3% among American Indians/Alaskan Natives, and 2% among Asians/Pacific Islanders (Table_1). Among non-Hispanic whites, the moderately preterm birthrate increased 10%, and minor changes were observed in very and extremely preterm birthrates. Among non-Hispanic blacks and Hispanics, the preterm birthrate decreased in the moderately, very, and extremely preterm subgroups (Table_1).

Maternal factors that may affect observed trends in preterm birthrates were analyzed. The percentage of singleton infants born to women aged greater than or equal to 35 years increased 43% (from 8.4% in 1989 to 12.0% in 1996), the percentage born to women who entered prenatal care during the first trimester increased 8% (from 75.6% to 81.8%), and the percentage born to unmarried women increased 20% (from 27.0% to 32.5%). Similar trends were observed in all racial/ethnic groups.

To control for changes in maternal factors, preterm birthrates were directly standardized for each racial/ethnic group to the combined 1989 and 1996 singleton live birth distributions for maternal age, time of entry into prenatal care, and marital status. After standardization, the change from 1989 to 1996 in the preterm birthrate among non-Hispanic whites was 3.8 per 1000 live-born infants, 37% lower than the crude rate change of 6.0 (Table_2). For other racial/ethnic groups, the standardized rate was lower than the crude rate by 50% among non-Hispanic blacks, 29% among Hispanics, and 78% among American Indians/Alaskan Natives.

In addition to changes in maternal factors, changes in obstetric practices occurred during the study period that may have influenced preterm birthrates. For example, the percentage of singleton infants born to women whose labor was medically induced increased from 9.1% to 17.1%. To determine whether changes in preterm birthrates were independent of the change in induction practices, medically induced births were excluded from the analysis and rates were again standardized for maternal age, marital status, and time of entry into prenatal care. In this restricted group, the standardized preterm birthrate increased 9% among non-Hispanic whites, decreased 4% among non-Hispanic blacks, and changed less than 2% among Hispanics, American Indians/Alaskan Natives, and Asians/Pacific Islanders.

The proportion of births for which gestational age estimates were based on clinical evaluation increased slightly during the study period (from 3.6% in 1989 to 4.7% in 1996). Because the method of determining gestational age may influence identification of a birth as preterm, an analysis was conducted that excluded births for which gestational age was clinically estimated. The standardized preterm birthrate for the study period increased 6.3% among non-Hispanic whites, decreased 5.0% among non-Hispanic blacks, and changed less than 2% among Hispanics, American Indians/Alaskan Natives, and Asians/Pacific Islanders.

Reported by: Div of Reproductive Health, National Center for Chronic Disease Prevention and Health Promotion; Div of Applied Public Health Training, Epidemiology Program Office; Div of Vital Statistics, National Center for Health Statistics; and an EIS Officer, CDC.

Editorial Note

Editorial Note: The findings in this report indicate that preterm birthrates among singletons are stable; however, the overall rate masks differences in trends by race/ ethnicity and among gestational age subgroups. The rate for singleton preterm births increased among non-Hispanic whites mainly because of an increase in the birthrate of moderately preterm infants. Among non-Hispanic blacks, the decline in moderately, very, and extremely preterm singleton births was substantial, and more modest declines were observed in overall preterm birthrates for Hispanics, American Indians/Alaskan Natives, and Asians/Pacific Islanders. The increase in singleton preterm birthrates among non-Hispanic whites and the decrease among non-Hispanic blacks are not explained entirely by changes in maternal age distribution, marital status, time of entry into prenatal care, induction rates, or use of clinical estimates of gestational age.

The findings in this study are subject to at least three limitations. First, LMP and clinical-based gestational age may be misclassified (e.g., because of imperfect maternal recall, postconception bleeding, delayed ovulation, or intervening early miscarriage); such errors may occur more frequently in some subpopulations, especially at shorter gestations (5). Second, changes in the reporting of preterm live births with the shortest gestations (ultra preterm) could have affected the preterm birthrates (6). However, these infants represent a small fraction of total preterm infants and do not contribute substantially to overall trends. Finally, because fetal deaths were not evaluated, the contribution of changes in fetal survival to the increase in preterm birthrates for non-Hispanic whites could not be assessed.

The disparity in preterm birthrates between blacks and whites is decreasing because of an increase in preterm births among non-Hispanic whites and a decrease among non-Hispanic blacks. The racial disparity in singleton preterm birth between non-Hispanic blacks and non-Hispanic whites decreased 17% from 1989 to 1996; however, in 1996, the risk for singleton preterm birth among blacks was still twice that for whites. Although many risk factors for preterm delivery have been identified, specific etiologies are not well characterized (7). In addition, many potential risk factors for preterm birth, such as urogenital tract infections (8) and history of subfertility or infertility (9) cannot be examined using the standard certificate of live birth. Additional studies exploring why preterm births are increasing among non-Hispanic whites and decreasing among non-Hispanic blacks may further understanding of how to prevent preterm birth.

References

1. Peters KD, Lochanek KD, Murphy SL. Deaths: final data for 1996. Hyattsville, Maryland: US Department of Health and Human Services, Public Health Service, CDC, National Center for Health Statistics, 1998. National Vital Stat Rep 1998;47(9).

2. Ventura SJ, Martin JA, Curtin SC, Mathews TJ. Report of final natality statistics, 1996. Mon Vital Stat Rep 1998;46(11, suppl).

3. Keith LG, Cervantes A, Mazela J, Oleszczuk JJ, Papiernik E. Multiple births and preterm delivery. Prenat Neonat Med 1998;3:125-9.

4. National Center for Health Statistics. Instruction manual part 12, Computer edits for natality data, 1989. Hyattsville, Maryland: US Department of Health and Human Services, CDC, 1991.

5. Alexander GR, Himes JH, Kaufman RB, Mor J, Kogan M. A United States national reference for fetal growth. Obstet Gynecol 1996;87:163-8.

6. Phelan ST, Goldenberg R, Alexander G, Cliver SP. Perinatal mortality and its relationship to the reporting of low-birthweight infants. Am J Public Health 1998;88:1236-9.

7. Berkowitz GS, Papiernik E. Epidemiology of preterm birth. Epidemiol Rev 1993;15:414-43.

8. Fiscella K. Racial disparities in preterm births. The role of urogenital infections. Public Health Rep 1996;111:104-13.

9. Henriksen TB, Baird DD, Olsen J, Hedegaard M, Secher NJ, Wilcox AJ. Time to pregnancy and preterm delivery. Obstet Gynecol 1997;89:594-9.

   
   
   Figure_1
   
   
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   Table_1
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   TABLE 1. Rate* of preterm+ birth among singleton infants, by maternal race/ethnicity&,
   gestational age group, and year -- United States, 1989 and 1996
   =====================================================================================
   Race/Ethnicity/
    Gestational age            1989      1996     % Change
   -------------------------------------------------------------------------------------
   Non-Hispanic white
       <20 weeks                0.1       0.1        7.7
     20-28 weeks                4.8       4.9        2.1
     29-32 weeks                9.9       9.9        0
     33-36 weeks               60.0      65.9        9.8
     Total                     74.8      80.8        8.0
   Non-Hispanic black
       <20 weeks                0.7       0.7        0
     20-28 weeks               20.5      19.1      - 6.8
     29-32 weeks               32.6      27.1      -16.9
     33-36 weeks              126.6     115.6      - 8.7
     Total                    180.4     162.5      - 9.9
   Hispanic
       <20 weeks                0.2       0.1      -23.5
     20-28 weeks                6.5       6.4      - 1.5
     29-32 weeks               14.5      13.4      - 7.6
     33-36 weeks               83.3      80.8      - 3.0
    Total                     104.5     100.7      - 3.6
   American Indian/           112.9     109.7      - 2.8
    Alaskan Native
   Asian/Pacific Islander      94.8      92.6      - 2.3
   All races
       <20 weeks                0.2       0.2      - 4.2
     20-28 weeks                7.6       7.3      - 3.9
     29-32 weeks               14.4      13.2      - 8.3
     33-36 weeks               74.8      76.5        2.3
     Total                     97.0      97.3        0.3
   -------------------------------------------------------------------------------------
   * Per 1000 live-born infants, rounded to the nearest tenth.
   + <37 completed weeks of gestation.
   & Stratification by gestational age was not performed for American Indians/Alaskan Natives and
     Asians/Pacific Islanders because the number of preterm births, when broken into gestational
     age subgroups, was too small for meaningful analysis.
   =====================================================================================
   

   
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   Table_2
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   TABLE 2. Crude and standardized rates* of preterm+ birth among sigleton infants and
   change in rate, by maternal race/ethnicity -- United States, 1989 and 1996
   ===================================================================================================
                                    Crude                                  Standardized&
                       ----------------------------------     ------------------------------------
                                             Change                                   Change
                        Rate               1989 to 1996            Rate            1989 to 1996
                       --------------    -----------------      ------------    ------------------
   Race/Ethnicity       1989     1996    Absolute    (%)        1989    1996    Absolute     (%)
   ---------------------------------------------------------------------------------------------------
   Non-Hispanic
    white               74.8     80.8      6.0     ( 8.0%)      81.4    85.2      3.8      ( 4.6%)
   Non-Hispanic
    black              180.4    162.5     17.9     (-9.9%)     154.6   145.6      9.0      (-5.8%)
   Hispanic            104.5    100.7      3.8     (-3.6%)      99.8    97.1      2.7      (-2.8%)
   American Indian/
    Alaskan Native     112.9    109.7      3.2     (-2.8%)     101.3   102.0      0.7      ( 0.7%)
   Asian/Pacific
    Islander            94.8     92.6      2.2     (-2.3%)     102.5    99.3      3.2      (-3.1%)
   ---------------------------------------------------------------------------------------------------
   * Per 1000 live-born infants.
   + <37 completed weeks of gestation.
   & Calculated by direct standardization using the combined 1989 and 1996 singleton live birth
     distributions for maternal age, entry into prenatal care, and marital status.
   ===================================================================================================
   

   
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