If the cluster is "unusual," then it is appropriate to consider possible causes for that cluster. The public health department would conduct a thorough search of the relevant literature. The objective of this literature review is to determine what is already known or hypothesized about the cluster. Have similar clusters been observed and, if so, how common are they? Have other individuals expressed concerns or documented associations with any environmental exposures? Have previously investigated clusters of the same or similar diseases yielded insights that might be useful for understanding the cluster concerns you? What is already well accepted about the causes of first-trimester miscarriages?