Structure and Control of Healthy Worker Effects in Studies of Pregnancy Outcomes.

Much of the healthy worker effect literature focuses on studies of chronic disease and mortality; however, when studying pregnancy outcomes, these effects might differ because of the short, defined risk periods of most pregnancy outcomes. Three pregnancy-specific healthy worker effects have also been described, but the structure of these effects have not yet been investigated when occupational exposure, and not employment status, is the exposure of interest. We used directed acyclic graphs to examine healthy worker effects in studies of occupational exposures and pregnancy outcomes: healthy hire effect, healthy worker survivor effect, desperation/privilege effect (differential workforce re-entry after pregnancy), reproductively unhealthy worker effect (women with live births leave the workforce, women with non-live births do not), and insecure pregnancy effect (women with adverse pregnancy outcomes reduce exposures in subsequent pregnancies). Given our assumptions, healthy hire effect, desperation/privilege effect, reproductively unhealthy worker effect, and insecure pregnancy effect resulted from confounding that can be addressed if measured confounders, such as employment status, are available. Presence of healthy worker survivor effect, however, varied by study design. Different types of healthy worker effects can be present in studies of occupational exposures and pregnancy outcomes, and many of them are easily addressed analytically.