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Fallacies in time to pregnancy studies


J. Olsen.Institute of Public Health, Department of Epidemiology, Aarhus University, Denmark and Department of Epidemiology, School of Public Health, UCLA, Los Angeles, USA

Time to pregnancy (TTP) as a research measure was probably first used by demographers to describe the biological capacity (fecundity) to reproduce in a population. They studied time from marriage to when the couple had their first child—a measure that is only useful in populations where marriage and sexual activity is closely linked. In populations that practice strict birth control and plan their pregnancies, it is, however, possible to record the time from “unprotected” sex to the time of pregnancy (or birth). Since these time periods are few in countries that have low fertility (the same countries that practice birth control), the amount of information available to be harvested in an epidemiological design is limited.

As it is often the case, the design of choice is a follow up design with prospectively collected data on occupational exposures before and during the time of trying. Such a study is expensive because a large group of couples have to be approached to get a reasonable sample size, unless one would settle for a cohort of pregnancy planners seeking infertility treatment. Such a group is of course not representative of the population of pregnancy planners at large, and the infertility treatment may well influence the exposures they control themselves.

The most frequently used design measure is TTP among pregnant women. This design can be done at low costs because pregnant women are easy to identify in most countries; however, the time to pregnancy measure includes only those who did get pregnant and the design is very sensitive to an equal compliance to the pregnancy attempt in the groups to be compared. One should furthermore recognise that the exposure information …

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