Preterm Singleton Births -- United States, 1989-1996 (2023)

Preterm birth (birth at less than 37 completed weeks of gestation) isthe second leading cause of neonatal mortality in the United States (1).Preterm birthrates differ by race; in 1996, black infants were 1.8 timesmore 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 otherhigher-order births) increased 19% (2). Multiple births are associated withpreterm birth (3); trends in preterm births independent of the influence ofmultiple births have not been fully explored. To characterize race- andethnicity-specific trends in preterm birth independent of multiple births,data from U.S. birth certificates for 1989-1996 were analyzed for singletonbirths only. This report summarizes the results of this analysis andindicates 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 occurringat 17-36 completed weeks of gestation and was subgrouped by weeks ofgestation: 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 byone of two methods (2,4): 1) the interval between the first day of themother's last normal menstrual period (LMP) and the date of birth, or 2) aclinical estimate by the birth attendant of gestational age when the monthor year of the LMP was missing or when the gestational age based on thisdate was inconsistent with the infant's birth weight. Approximately 1% ofsingleton infants were excluded because of missing or implausible estimatesof gestational age. Infants were imputed as singletons for the 0.02% oflive-born infants for which the number of fetuses in a given pregnancy wasunreported. Maternal race and ethnicity were based on self-report andcategorized as non-Hispanic white, non-Hispanic black, Hispanic, AmericanIndian/Alaskan Native, or Asian/Pacific Islander. Stratification bygestational age was not performed for American Indians/Alaskan Natives andAsians/Pacific Islanders because the number of preterm births, when brokendown into gestational age subgroups, was too small for meaningful analysis.

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

Maternal factors that may affect observed trends in preterm birthrateswere analyzed. The percentage of singleton infants born to women agedgreater 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 thefirst trimester increased 8% (from 75.6% to 81.8%), and the percentage bornto unmarried women increased 20% (from 27.0% to 32.5%). Similar trends wereobserved in all racial/ethnic groups.

To control for changes in maternal factors, preterm birthrates weredirectly standardized for each racial/ethnic group to the combined 1989 and1996 singleton live birth distributions for maternal age, time of entryinto prenatal care, and marital status. After standardization, the changefrom 1989 to 1996 in the preterm birthrate among non-Hispanic whites was3.8 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 waslower than the crude rate by 50% among non-Hispanic blacks, 29% amongHispanics, and 78% among American Indians/Alaskan Natives.

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

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

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

Editorial Note

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

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, delayedovulation, or intervening early miscarriage); such errors may occur morefrequently in some subpopulations, especially at shorter gestations (5).Second, changes in the reporting of preterm live births with the shortestgestations (ultra preterm) could have affected the preterm birthrates (6).However, these infants represent a small fraction of total preterm infantsand do not contribute substantially to overall trends. Finally, becausefetal deaths were not evaluated, the contribution of changes in fetalsurvival to the increase in preterm birthrates for non-Hispanic whitescould not be assessed.

The disparity in preterm birthrates between blacks and whites isdecreasing because of an increase in preterm births among non-Hispanicwhites and a decrease among non-Hispanic blacks. The racial disparity insingleton preterm birth between non-Hispanic blacks and non-Hispanic whitesdecreased 17% from 1989 to 1996; however, in 1996, the risk for singletonpreterm birth among blacks was still twice that for whites. Although manyrisk factors for preterm delivery have been identified, specific etiologiesare not well characterized (7). In addition, many potential risk factorsfor preterm birth, such as urogenital tract infections (8) and history ofsubfertility or infertility (9) cannot be examined using the standardcertificate of live birth. Additional studies exploring why preterm birthsare increasing among non-Hispanic whites and decreasing among non-Hispanicblacks 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 8.0Non-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 - 9.9Hispanic <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 - 3.6American Indian/ 112.9 109.7 - 2.8 Alaskan NativeAsian/Pacific Islander 94.8 92.6 - 2.3All 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 andchange 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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