The analysis of recurrence risk as an epidemiological tool.

Recurrence risk provides information on the heterogeneity of risk in the population, and thus is useful for aetiological studies. While recurrence risk is observable in many areas of epidemiology, it is particularly accessible in the study of perinatal events. High recurrence rates of pregnancy problems suggest genetic causes, but can also reflect the presence of persistent environmental causes. Specific patterns of recurrence risk can provide further clues about the relative importance of genetic vs. environmental factors, for example through comparisons of recurring pregnancy problems in women who change their male partner and women who keep the same partner. Interpretation of recurrence risk is subject to confusion from bias and confounding, and some examples are discussed. In addition to providing information on causality, recurrence risk offers a means for more efficient design of aetiological studies.