Temporal Trends in the Prevalence of Congenital Malformations at Birth Based on the Birth Defects Monitoring Program, United States, 1979-1987

Larry D. Edmonds, M.S.P.H. Levy M. James, M.S. Birth Defects and Genetic Diseases Branch Division of Birth Defects and Developmental Disabilities Center for Environmental Health and Injury Control

Summary The Birth Defects Monitoring Program (BDMP) is a national program that monitors congenital malformations by using hospital discharge data concerning newborns. BDMP data for 38 selected malformations were examined for temporal trends. The prevalence at birth of these malformations for the two periods 1979-1980 and 1986-1987 was compared and then grouped into three time-trend categories: increasing greater than or equal to 2% annually, decreasing greater than or equal to 2% annually, and remaining stable. Results showed that 29 malformations had increasing trends, two had decreasing trends and seven remained stable. Follow-up studies are needed to evaluate whether these trends are real or due to improved ascertainment of the respective malformations. INTRODUCTION

The Birth Defects Monitoring Program (BDMP) is a national program that monitors congenital malformations by using hospital discharge data concerning newborns (1). The BDMP was initiated at CDC in December 1974. Data used in this program are derived from information sent by participating hospitals to the Commission on Professional and Hospital Activities (CPHA) as part of its health-data processing system. Discharge abstracts are coded by hospital staff of the medical records department and submitted to CPHA for processing. CPHA provides a subset of these data to CDC. Included are abstracts on all live and stillborn infants delivered in each participating CPHA hospital. The data are reviewed two times per year, and malformations are usually reported 3-6 months after an affected infant's birth. Although this data source is not population based and not a random sample of U.S. births, it nevertheless represents the largest single set of uniformly collected and coded discharge data on congenital malformations in the United States.

The BDMP functions primarily as an early warning system; however, it can be useful also for correlating incidence patterns with such trends as the temporal and geographic distribution of drugs, chemicals, and other possible human teratogens. It is one of the few data bases currently available that enables researchers to review long-term trends in the prevalence at birth of congenital malformations in the United States. The current BDMP data base contains data from 1970 to the present on 15 million births that have occurred in 1,200 predominantly mid-sized community hospitals. In 1987, 575,000 births (15% of U.S. births) were monitored. METHODS

The BDMP data are routinely analyzed for 161 categories of birth defects to identify increases, decreases, or other unusual patterns of occurrence. For this analysis, temporal trends for 38 malformations were examined for the period from 1979 to 1987. The starting year of 1979 was chosen because that was the year when the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes for such defects as fetal alcohol syndrome (FAS) were included in the data base. The 38 malformations were selected because they a) occur in sufficient numbers to provide relatively stable rates, b) affect different organ systems, and c) are of sufficient severity to be of public health concern.

Total prevalence at births of these malformations for the two periods (1979-1980 and 1986-1987) were compared. For each defect, the geometric mean annual percentage change in rate was computed for the 7 years between the midpoints of these two periods. A defect was arbitrarily categorized as increasing if its mean annual rate change was at least 2%. Likewise, a defect with a mean annual percentage change of -2% or more negative was categorized as decreasing. RESULTS

On the basis of the above criteria, 29 malformations had increasing trends, two (anencephalus and spina bifida) had decreasing trends (Figures 1 and 2), and seven remained stable (Table 1). Of the increases, renal agenesis (Figure 3) was studied further (2). Almost half of the malformations with increasing trends were cardiac defects. Although the increases in these cardiac defects are dramatic, the increased rates may be the result of improvements in health care and advances in diagnostic techniques. These developments could result in the diagnosis of more mild or asymptomatic defects as well as those serious defects in infants who otherwise may never have survived until diagnosis. In addition, many of these diagnoses in the BDMP may be provisional, because these data are for newborns only. Plans are under way to evaluate the data on cardiac defects and to define the role of improved health care and new diagnostic techniques. DISCUSSION

Interpretation of birth defects occurrence trends is difficult because of the several factors that may influence reporting. These include ease, precision, and uniformity of diagnosis; classification, coding, and reporting; and the infant's age at the time the defect is usually recognized. Changes in the rates of defects readily apparent at birth, such as anencephaly and spina bifida, are more likely to be actual changes than those reported for renal or heart defects, for example, because diagnosis of the latter defects requires more careful clinical examinations or special diagnostic techniques.

Although the BDMP rates for cardiac defects increased, none of the rates in 1986-1987 exceeded the rates for the same defects in the Metropolitan Atlanta Congenital Defects Program (MACDP), an active surveillance system that identifies case infants up to 1 year of age.

One increasing trend that has been studied in more detail is the increase in birth prevalence of renal agenesis (2). The descriptive study indicated that, in the United States, the birth prevalence of renal agenesis fluctuated during the period 1970-1982 and peaked in 1975, although the overall rate did not increase during the 13-year period. That study also indicated that the birth prevalence of bilateral renal dysgenesis increased steadily from 1970 to 1987. Because of changes in nosologic coding procedures, the increase in renal dysgenesis is difficult to interpret. However, no strong evidence has been found to suggest that the increase is an artifact.

Increases in other defect categories such as FAS and Trisomy 18 may also have been the result of better clinical awareness and recognition of these malformations. Two noteworthy decreasing categories are anencephalus and spina bifida. Possible reasons for these decreases are a) the impact of prenatal diagnosis, particularly on anencephalus, and b) maternal lifestyle changes, such as better nutrition, which may decrease the risk of these defects. Several reports have suggested that the use of periconceptional multivitamins may contribute to the reduction of the birth prevalence of anencephaly and spina bifida (3,4). Although prenatal diagnosis is undoubtedly having an impact on the current prevalence at birth of neural tube defects, the declining trend of neural tube defects predates the widespread use of prenatal diagnosis in the United States. This finding suggests that other environmental factors may be affecting the occurrence of these birth defects.

References

1. Edmonds LD, Layde PM, James LM, Flynt JW Jr, Erickson JD, Oakley GP Jr. Congenital malformations surveillance: two American systems. Int J Epidemiol 1981;10:247-52.

2. CDC. Renal agenesis surveillance--United States. MMWR 1988;37:679-86.

3. Mulinare J, Cordero JF, Erickson JD, Berry RJ. Periconceptional use of multivitamins and the occurrence of neural tube defects. JAMA 1988;260:3141-5.

4. Milunksy A, Jick H, Jick SS, Bruell CL, MacLaughlin DS, Rothman KJ, Willett W. Multivitamins/folic acid supplementation in early pregnancy reduces the prevalence of neural tube defects. JAMA 1989;262:2847-52.

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