Preterm Delivery Rates in North Carolina: Are They Really Declining among Non-Hispanic African Americans?

Anjel Vahratian1  , Pierre Buekens1, Trude A. Bennett1, Robert E. Meyer2, Michael D. Kogan3 and Stella M. Yu3

1 Department of Maternal and Child Health, University of North Carolina at Chapel Hill, Chapel Hill, NC.
2 State Center for Health Statistics, Division of Public Health, North Carolina Division of Health and Human Services, Raleigh, NC.
3 Office of Data and Information Management, Maternal and Child Health Bureau, Rockville, MD.

Received for publication February 12, 2003; accepted for publication July 15, 2003.


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The preterm delivery rate in North Carolina is consistently higher than the national average. However, recent reports suggest that singleton preterm delivery rates for non-Hispanic Whites are increasing while those for non-Hispanic African Americans are decreasing. To study this pattern further, the authors examined data on singleton non-Hispanic White and non-Hispanic African-American births in 1989 and 1999 by using North Carolina vital statistics data. They found that the frequency of preterm delivery rose 1.1% (8.5% to 9.6%) among non-Hispanic Whites but declined 1.4% (17.9% to 16.5%) among non-Hispanic African Americans over the same time period. For both subgroups, a bimodal distribution of birth weights was apparent among preterm births at 28–31 weeks of gestation. The second peak with its cluster of normal-weight infants was more prominent among non-Hispanic African Americans in 1989 than in 1999. To reduce the potential for bias due to misclassification of infant gestational age, frequencies of preterm delivery of infants who weighed less than 2,500 g were calculated. Unlike the original analysis, this calculation showed that preterm delivery increased for both subgroups. A number of non-Hispanic African-American births classified as preterm were apparently term births mistakenly assigned short gestational ages. Such misclassification was more frequent in 1989 than in 1999, inflating 1989 preterm delivery rates.

bias (epidemiology); birth certificates; blacks; classification; delivery, obstetric; gestational age; infant, premature


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Published data suggest that the disparity in preterm delivery rates is narrowing between non-Hispanic African-American and non-Hispanic White women in the United States. Between 1990 and 1997, the singleton preterm delivery rate decreased 10 percent among non-Hispanic African-American women but increased 11 percent among non-Hispanic White women (1). While the increase for non-Hispanic Whites was significant in 38 states, the positive trends for non-Hispanic African Americans were more variable; only 24 states had significant declines in the rates of preterm deliveries of infants of non-Hispanic Black mothers (1).

In North Carolina, a southeastern state with historically high rates of infant mortality and other adverse infant health outcomes, a similar phenomenon seems to be operating. Data from US natality files for 1990 and 1997 indicate that for North Carolina, the singleton preterm delivery rate among non-Hispanic Whites increased from 83.9 to 91.7 per 1,000 livebirths while the preterm delivery rate among non-Hispanic African Americans declined from 188.2 to 165.9 per 1,000 livebirths (1). The reasons for such trends are not fully understood. The Centers for Disease Control and Prevention (Atlanta, Georgia), using a nationally representative sample of livebirths, reported a similar decline in preterm delivery rates among non-Hispanic African Americans (2). The authors acknowledged that their findings for non-Hispanic African Americans could be due to errors in recording the date of a woman’s last menstrual period, which may have resulted in misclassification of gestational age and thus preterm status. However, the authors did not assess for misclassification of gestational age in their analysis. Therefore, the present study sought to identify possible explanations for changes in preterm delivery rates in North Carolina.


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
For this analysis, we examined data from 1989 and 1999 North Carolina electronic livebirth files. Our sample consisted of singleton non-Hispanic White and non-Hispanic African-American infants born in 1989 (n = 97,298) and 1999 (n = 98,614). Preterm delivery was the main outcome evaluated, defined as a livebirth at 18–36 weeks of gestation. Infant race/ethnicity was determined by both the reported race of the mother and her Hispanic origin status.

We first conducted a descriptive analysis of the data and calculated preterm delivery rates stratified by infant race. In the next step, we calculated birth-weight distributions within strata of gestational age to explore possible misclassifications of gestational age. Gestational age was computed on the basis of the date of a woman’s last menstrual period. If this information was missing or implausible, the clinical estimate of gestational age was used in its place and was defined broadly as the number of weeks of gestation based on the clinical evidence available. Because the date of the last menstrual period is not included in the North Carolina public use electronic livebirth files, it was not possible to compare preterm delivery rates based on the date of the last menstrual period and rates based on the gestational age measurement included in the electronic livebirth file. However, the North Carolina State Center for Health Statistics reports that the clinical estimate was used for 4.6 percent of the births to non-Hispanic Whites and for 7.4 percent of the births to non-Hispanic African Americans. Misclassification of gestational age among normal birth weight infants can be detected by a bimodal distribution of weights among preterm infants (35). Thus, as a means of excluding infants whose gestational ages were potentially misclassified, we calculated preterm delivery rates among infants who weighed less than 2,500 g.

Lastly, we used the Wilcox-Russell approach to separate birth-weight distributions of all births into predominant and residual (6, 7). The predominant distribution is bell shaped and can reflect the birth-weight distribution of term births. The residual distribution represents births in the lower tail of the curve outside the predominant distribution and thus the proportion of small and preterm births. Together, the predominant and residual distributions of birth weight allow for insights into gestational age without requiring calculated gestational age data. This approach avoids misclassification of gestational age by using the frequency distribution of birth weight.

All analyses were performed by using Statistical Analysis Software (SAS), version 8.1 (SAS Institute, Inc., Cary, North Carolina).


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
For non-Hispanic Whites, the preterm delivery rate increased from 8.5 percent in 1989 to 9.6 percent in 1999 (table 1). However, a different trend appeared for non-Hispanic African Americans. Specifically, the preterm delivery rate decreased from 17.9 percent in 1989 to 16.5 percent in 1999. Most of the changes in preterm delivery rates for both subgroups occurred in the moderately preterm category (32–36 weeks).


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TABLE 1. Number and frequency of preterm births, according to gestational-age categories, to non-Hispanic White and non-Hispanic African-American women, North Carolina, 1989 and 1999
 
Figure 1 highlights the distribution of birth weights at less than 28, 28–31, 32–35, and 36 weeks of gestation for non-Hispanic White and non-Hispanic African-American singleton infants. At 28–31 weeks, the distribution of birth weights was distinctly bimodal for both groups. The second peak was more prominent in 1989 among the infants weighing 2,600–3,000 g. However, the first peak was larger in 1999 for the non-Hispanic African-American infants who weighed 1,200–1,800 g.



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FIGURE 1. Birth-weight distributions for non-Hispanic White and non-Hispanic African-American newborns at <28, 28–31, 32–35, and 36 weeks of gestation, North Carolina, 1989 and 1999.

 
A bimodal distribution of birth weights among preterm infants strongly suggests misclassification of gestational age. While misclassification existed for preterm infants at 28–31 weeks of gestation during both time periods and for both subgroups, it was more pronounced for non-Hispanic African Americans in 1989 (figure 1). Errors in measuring gestational age may be due to factors such as menstrual cycle irregularities, early vaginal bleeding during pregnancy, or inaccurate recall of when the last menstrual period started (8, 9). Unfortunately, we were unable to assess the validity of these possible explanations because this information is not recorded in vital records. However, we were able to determine whether the misclassification at 28–31 weeks was associated with changes in the regionalization of care, early initiation of prenatal care, or increased use of ultrasound. In North Carolina, 12 hospitals are considered tertiary care centers (i.e., level III hospitals). The proportion of infants who weighed 2,500 g or more and were delivered in tertiary care centers remained constant from 1989 to 1999 (table 2). However, compared with non-Hispanic White infants, more non-Hispanic African-American infants weighing 2,500 g or more were born in tertiary care centers across both time periods. Thus, the misclassification of gestational age does not appear to be explained by changes in regionalization of care. Although more infants weighing 2,500 g at 28–31 weeks of gestation are born in nontertiary care centers, the proportion itself remained constant for both subgroups across both time periods.


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TABLE 2. Characteristics (%) of singleton infants aged 28–31 weeks, according to birth-weight status, North Carolina, 1989 and 1999
 
Early initiation of prenatal care may be associated with a more reliable estimate of gestational age. In this subset of births, non-Hispanic White women initiated prenatal care earlier than did non-Hispanic African-American women. Moreover, across both time periods, the proportion of women who began prenatal care in the first trimester was higher among those who delivered infants weighing less than 2,500 g compared with women whose infants weighed 2,500 g or more. These findings suggest that the misclassification could be related to a later entry into prenatal care for non-Hispanic African-American women in 1989.

The availability and use of ultrasound technology during pregnancy has increased over the past decade and may be associated with improvements in calculating gestational age. In North Carolina, the proportion of women who delivered an infant at between 28 and 31 weeks and received at least one ultrasound examination during pregnancy increased from 1989 to 1999 for both non-Hispanic Whites and non-Hispanic African Americans (table 2). However, non-Hispanic White women were more likely to have an ultrasound examination during pregnancy across both time periods, which suggests that there may be less misclassification of gestational age among non-Hispanic White infants delivered at between 28 and 31 weeks of gestation. This finding is supported by the data presented in figure 1. Birth certificates record only whether the woman received an ultrasound examination during pregnancy. Thus, information is not available on the timing of the examination or whether it was used to determine the clinical estimate of gestational age.

Our findings suggest that of those preterm infants whose recorded birth weight was more than 2,500 g, more non-Hispanic White infants were delivered at 35–36 weeks than non-Hispanic African-American infants, who were more likely to be delivered at 28–34 weeks (data not shown). It is not unusual for infants born at 35–36 weeks to weigh more than 2,500 g; these infants were probably closer to their correct gestational age rather than misclassified. However, it is less likely for an infant delivered at 28–34 weeks to weigh more than 2,500 g. Thus, these infants are more likely to have been misclassified. Since non-Hispanic African-American infants were more likely than non-Hispanic White infants to be in this category, it is likely that misclassification of gestational age was greater among non-Hispanic African Americans than among non-Hispanic Whites.

Excluding preterm infants whose birth weights were more than 2,500 g reduced the percentage of preterm births by approximately 60 percent for non-Hispanic Whites and by about 50 percent for non-Hispanic African Americans. After excluding preterm infants who weighed 2,500 g or more, we found that the preterm delivery rate for non-Hispanic African Americans actually increased from 1989 to 1999 (8.4 percent and 8.9 percent, respectively). The preterm delivery rate for non-Hispanic Whites increased from 3.2 percent to 3.9 percent from 1989 to 1999, respectively. As an alternative method of estimating the proportion of small, preterm births in both subgroups, we computed the residual distribution of birth weight proposed by Wilcox and Russell (6, 7). This analysis of the birth-weight distributions indicated that the residual distribution of birth weight increased for both subgroups over the two time periods (table 3). From 1989 to 1999, the residual distribution for non-Hispanic Whites increased from 2.4 percent to 2.8 percent. Among non-Hispanic African Americans, the residual distribution increased from 4.6 percent to 5.3 percent over the same period. Thus, for both subgroups, the proportion of small, preterm births increased from 1989 to 1999.


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TABLE 3. Residual and predominant distributions of birth weight* for non-Hispanic White and non-Hispanic African American newborns, North Carolina, 1989 and 1999
 

    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
This study examined trends in preterm delivery among non-Hispanic Whites and non-Hispanic African Americans in North Carolina from 1989 to 1999. Our findings suggest that preterm delivery rates increased for both non-Hispanic Whites and non-Hispanic African Americans. Moreover, the decline in preterm deliveries among non-Hispanic African Americans, as reported in earlier studies, appears to have been influenced by misclassification of gestational age.

To our knowledge, this study is the first published report of an increase in preterm births over the past decade among non-Hispanic African Americans in North Carolina. An analysis by Centers for Disease Control and Prevention researchers, using a nationally representative sample of livebirths, reported a decline in preterm delivery rates among non-Hispanic African Americans (2). The authors acknowledged that their findings for non-Hispanic African Americans could be due to errors in recording the date of a woman’s last menstrual period, which may have resulted in misclassification of gestational age and thus preterm status. However, their analysis does not appear to have assessed the potential for misclassification of gestational age. Rather, the authors indicated that changes in factors such as the distribution of maternal age, marital status, medical induction of labor, and entry into prenatal care could partially explain the observed trends. Future publications on trends in singleton preterm delivery rates may benefit from taking potential misclassification into account when interpreting the data.

In an analysis of US birth and infant death files from 1989 to 1997, Demissie et al. (10) also reported a reduction in preterm birth rates among African Americans. Earlier access to prenatal care and reductions in tobacco use during pregnancy were said to have had favorable effects on African Americans but were unlikely to explain the differences in preterm delivery rates between the two race groups. Rather, the authors concluded that unmeasured factors, such as economic improvement, could explain reductions in preterm delivery rates among African-American women.

Because gestational age estimates are prone to misclassification because of errors in maternal self-report of the date of the last menstrual period, alternate statistical measures have been developed to provide more accurate birth-weight distributions for gestational age. Approximately 20 years ago, Wilcox and Russell (6, 7) proposed the use of predominant and residual distributions of birth weight as an indirect method of presenting information on gestational age without actually using gestational age data. Although the residual distribution does not correspond to the proportion of all preterm births, it does identify those are at the highest risk: small preterm births. In our analysis, the residual distribution for both non-Hispanic Whites and non-Hispanic African Americans increased from 1989 to 1999, supporting the argument that public health interventions aimed at primary prevention of preterm birth have not been successful.

In conclusion, this analysis demonstrated that a number of non-Hispanic African-American births classified as preterm were apparently term births mistakenly assigned short gestational ages. Such misclassification appeared to be more frequent in 1989 than in 1999, inflating 1989 preterm delivery rates. Thus, it seems that the misclassification of gestational age, not the preterm delivery rate, has declined for non-Hispanic African Americans in North Carolina.


    ACKNOWLEDGMENTS
 
This work was supported in part by US Maternal and Child Health Bureau contract 240-97-0028.


    NOTES
 
Presented in part at the 15th Annual Meeting of the Society for Pediatric and Perinatal Epidemiologic Research, Palm Desert, California, June 17–18, 2002. Back

Reprint requests to Anjel Vahratian, Division of Epidemiology, Statistics, and Prevention Research, National Institute of Child Health and Human Development, National Institutes of Health, Department of Health and Human Services, 6100 Executive Boulevard, Room 7B03, Bethesda, MD 20892 (e-mail: vahratia{at}mail.nih.gov). Back


    REFERENCES
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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