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cluster investigation
exposure
small area study
teratogen


The California Birth Defects Monitoring Program is capable of springing into action when there is concern that local environmental conditions may be linked to birth defects. Small area investigations can be reactive or proactive, responding to community cluster reports or an environmental emergency.

INVESTIGATING BIRTH DEFECTS CLUSTERS

A cluster is a mini-epidemic, where too many affected babies are born in a short time span. Birth defects, like other health conditions, often occur in clusters. Most happen by coincidence or chance alone. Others can be explained by changes in medical practices. Very rarely, a cluster is due to a teratogen—an environmental exposure that causes birth defects.

IN THE AFTERMATH OF AN ENVIRONMENTAL CRISIS

Occasionally, an environmental emergency prompts concerns. For example, in 1991, The Program tracked over 100 pregnancies conceived shortly before or after a massive spill of the toxic herbicide metam-sodium into the Sacramento River. (We determined that birth defects were not elevated.)

HALLMARKS OF A TERATOGEN

A local investigation can only detect “sledgehammers”--very potent teratogens. Otherwise, the limited numbers involved aren’t statistically powerful enough to resolve questions. At best, small area studies generate clues—even if results are suspicious, they must be confirmed with a larger case-control study.

In evaluating community concerns, we look for the features noted in scenarios where teratogens have been involved:

bullet cases with the same or developmentally related birth defects
bullet a large increase (more than 10 times the expected rate)
bullet an exposure in common.

Investigating Birth Defects Clusters: A Systematic Approach





REFINEMENTS OVER TIME

When the Program began, we hoped that cluster investigations would generate clues about birth defects causes. But after investigating more than 200 clusters—which raised anxiety but rarely provided answers—we identified no teratogens. We re-examined our strategy and found:

bullet Random fluctuation probably accounts for most rate variation in small populations.
bullet Small numbers have no statistical power. Even if a teratogen is present, it would have to increase risks dramatically to be detected.
bullet Research studies are the best way to identify causes. When there is an environmental concern—such as pesticides—large-scale, rigorous studies are more informative than drawing conclusions from a small area. Research can identify the “sledgehammers” and pinpoint subtle risk factors.

SMALL AREA INVESTIGATIONS
Protocol
bullet Investigating Birth Defects Clusters: A Systematic Approach
Local & Regional Investigation Results
bullet Monitoring for Birth Defects Following the Cantara Loop Spill (1992)
bullet Neural Tube Defects in Kern County: Buttonwillow Area Cluster Investigation, Summary and Full Report (1993)
bullet Birth Defects in Lompoc (1996)
  Birth Defects in McFarland (1996)
  Birth Defects in Bayview/Hunter's Point (1997)
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