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Surveillance for and Comparison of Birth Defect Prevalences in Two Geographic Areas -- United States, 1983-88

SUGGESTED CITATION: General: Centers for Disease Control and Prevention. CDC Surveillance Summaries, March 19, 1993. MMWR 1993;42:(No. SS-1)

Specific: Centers for Disease Control and Prevention. {Title of particular article.} In: CDC Surveillance Summaries, March 19, 1993. MMWR 1993;42:(No. SS-1):{inclusive page numbers}.

CIO Responsible for this publication: National Center for Environmental Health


Problem/Condition: CDC and a number of states have developed surveillance systems to monitor the birth prevalence of major defects.

Reporting Period Covered: This report covers birth defects surveillance in Metropolitan Atlanta, Georgia and selected jurisdictions in California for the years 1983-1988. Description of System: The California Birth Defects Monitoring Program and the Metropolitan Atlanta Congenital Defects Program are two population based surveillance systems that employ similar data collection methods. The prevalence estimates for 44 diagnostic categories are based on data from 1983 to 1988 for 639,837 births in California and 152,970 births in metropolitan Atlanta. The prevalences in the two areas are compared adjusting for race, sex and maternal age using Poisson regression. Results: Regional differences in the prevalence of aortic stenosis, fetal alcohol syndrome, hip dislocation/dysplasia, microcephalus, obstruction of the kidney/ureter, and scoliosis/lordosis may be attributable to general diagnostic variability. However, differences in the prevalences of arm/hand limb reduction, encephalocele, spina bifida, or trisomy 21 (Down Syndrome) are probably not attributable to differences in ascertainment because these defects are relatively easy to diagnose.

Interpretation: Regional differences in prenatal diagnosis and pregnancy termination may affect prevalences of trisomy 21 and spina bifida. However, the reason for differences in arm/hand limb reduction is unknown, but may be related to variability in environmental exposure, heterogeneity in gene pool, or random variation.

Actions Taken: Because of the similarities of these data bases, several collaborative studies are being implemented. In particular, the differences in the birth prevalence of spina bifida and Down Syndrome will focus attention on the impact of prenatal diagnosis.


Many states have begun birth defects monitoring programs in response to growing public concern about the potential effects of environmental hazards (1). The data in this surveillance summary are from one of the largest (California Birth Defects Monitoring Program {CBDMP}) and one of the oldest (Metropolitan Atlanta Congenital Defects Program {MACDP}) birth defects registries in the United States. This study was possible because of the existence of these two population-based registries that employ similar data collection methods. The prevalence of specific birth defects in two geographic areas was compared because studying the geographic distribution of disease may be important in the search for possible etiologic clues.


The CBDMP -- instituted in five San Francisco Bay Area counties in 1983 -- initially monitored about 75,000 births annually. Currently, more than 300,000 births per year are monitored. Data collection specialists routinely visit all hospitals and genetic centers to identify children less than 1 year of age who are diagnosed with major structural malformations. The medical charts for these children are reviewed and detailed demographic and diagnostic information is abstracted (2).

The MACDP, which is directed by CDC and sponsored by CDC, the Emory University School of Medicine, and the Georgia Mental Health Institute, has maintained a population-based registry of malformed children since October 1967. Approximately 40,000 births occur each year in the MACDP surveillance area. Like the CBDMP, the MACDP includes an active surveillance component; data collection specialists visit hospitals to identify records of children with birth defects (3). Only slight differences exist between the CBDMP and the MACDP in how children's records are identified for chart review and in the criteria for diagnostic inclusion.

Diagnostic information for this report is grouped into 44 categories on the basis of a classification system developed at the MACDP. The categories represent a variety of anatomic abnormalities and reflect the malformations most frequently addressed in scientific literature.

Prevalence estimates were based on data from 1983 to 1988 for 639,897 births in California (561,737 whites and 78,160 blacks), and 152,970 births in metropolitan Atlanta (96,380 whites and 56,590 blacks). Race was defined using maternal race as designated on the birth certificate. Because few Asians and Hispanics reside in Atlanta, data for these groups were excluded in this analysis. Only data for live births were analyzed.

Because the racial composition of the two areas differs and the rates of some birth defects vary by race, the prevalence of each defect by race and by region was estimated separately. Exact 95% confidence intervals (CIs) for the estimates are presented (4).

The ratio of the prevalence for the MACDP compared to the CBDMP in each defect category was also estimated. Values greater than 1.0 indicate that the prevalence is higher for the MACDP than for the CBDMP. These ratios have been adjusted for sex, maternal age, and race using Poisson regression (5). Approximate 95% CIs for the ratios are also presented.


Prevalence estimates are presented separately for whites and blacks in each geographic area (Table 1). Only those categories that have at least a two-fold difference in the prevalence of disease between the two areas are mentioned in the table, and at least four cases of disease for each race in each area. Only one of the 44 diagnostic categories, microcephalus, met these criteria for both races simultaneously; the prevalence for microcephalus was higher in the CBDMP than in the MACDP. The prevalence for stenosis/atresia of the duodenum among blacks and for scoliosis/lordosis among whites was higher in the CBDMP when compared with the MACDP. Conversely, the prevalence for spina bifida and encephalocele among blacks was higher in the MACDP than in the CBDMP.

When the potential confounding effects of maternal age, sex, and race were removed, the prevalence of disease in the two geographic areas was similar for most birth defect categories examined. However, the CBDMP has a higher prevalence of arm/hand limb reduction, microcephalus, obstruction of the kidney/ureter, scoliosis/lordosis, hip dislocation/dysplasia and fetal alcohol syndrome. The MACDP has a higher prevalence of spina bifida, aortic stenosis, encephalocele, and trisomy 21 (Down's Syndrome).

Data for stillbirths are not included in this analysis because the enumeration of fetal deaths (in California) is incomplete and the diagnosis of birth defects among fetal deaths is not consistent. Although estimates for both areas are higher for certain malformations (e.g., anencephalus) when stillbirths are included in the calculations, the patterns observed do not change.


The prevalences of spina bifida, encephalocele, and arm/hand limb reduction vary distinctively in the two populations described in this surveillance report. In addition, these two populations differ in the rates of anencephalus, hypoplastic left heart, and single ventricle. Because all of these defects are easy to identify and diagnose, it is unlikely that these differences are due to differences in ascertainment. Anencephalus, spina bifida, and encephalocele are all defects of the neural tube; the Metropolitan Atlanta rates were notably elevated compared to the California rates for all three of these defects. Other reports have noted a higher prevalence of spina bifida and anencephalus in the eastern states compared with the western states (6). In addition, regional differences in prenatal diagnosis and early termination of pregnancy may account for some of the differences in the rates for trisomy 21, but information is not available to assess the potential impact of these practices.

The reasons for the observed differences between these populations for arm/hand limb reduction, hypoplastic left heart, and single ventricle are unknown. The differences in these defects are possibly due to regional factors, environmental exposures, the genetic makeup of the two populations, or random variations. Regional differences in prenatal diagnosis and early termination of pregnancy may account for some of the differences in the rates for anencephalus, spina bifida, and encephalocele.

Statistically significant differences exist between the two populations in the prevalences of several conditions (including microcephalus, aortic stenosis, obstruction of the kidney or ureter, hip dislocation/dysplasia, scoliosis/lordosis, fetal alcohol syndrome, and a marginally significant difference for tetralogy of Fallot); however, these conditions are more difficult to diagnose. Therefore, the regional differences in these defects are possibly due to variability in case ascertainment. However, regional differences in environmental exposures or differences in genetic makeup of the populations cannot be discounted.

Although some differences are noted, the two systems are very similar. Similarities have led to a number of collaborative studies. This analysis has also pointed out the need to pay attention to regional differences in prenatal diagnosis when analyzing data.


  1. Flynt JW, Norris CK, Zarro S, Kitchen SB, Kolter M, Ziegler A. Final report, state surveillance of birth defects and other adverse reproductive outcomes. Washington, D.C.: U.S. Department of Health and Human Services, 1987.

  2. Croen L, Schulman J, Roeper P. Birth defects in California, January 1, 1983-December 31, 1986: CBDMP report series. Emeryville, CA: California Birth Defects Monitoring Program, 1990;BDR3(90):48.

  3. Edmonds LD, Layde PM, James LM, Flynt JW, Erickson JD, Oakley G. Congenital malformation surveillance: two American systems. Int J Epi 1981;10(3):247-52.

  4. Blyth CR. Approximate binomial confidence limits. JASA 1986;81(395):843-55.

  5. Kleinbaum DG, Kupper LL, Muller KE. Applied regression analysis and other multivariable methods. Boston, MA: PWS-Kent Publishing Company, 1988.

  6. Elwood JM, Elwood JH. Epidemiology of anencephalus and spina bifida. Oxford: Oxford University Press, 1980.

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