American Journal of Epidemiology Advance Access originally published online on June 22, 2005
American Journal of Epidemiology 2005 162(2):115-124; doi:10.1093/aje/kwi172
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Studying Time to Pregnancy by Use of a Retrospective Design
1 Department of Epidemiology and Public Health, School of Medicine, Imperial College, London, United Kingdom
2 Department of Biostatistics, University of Copenhagen, Copenhagen, Denmark
3 University of Southern Denmark, Odense, Denmark
Correspondence to Dr. Michael Joffe, Department of Epidemiology and Public Health, School of Medicine, Imperial College, St. Mary's Campus, Norfolk Place, London W2 1PG, United Kingdom (e-mail: m.joffe{at}imperial.ac.uk).
Biologic fertility can be measured using time to pregnancy (TTP). Retrospective designs, although lacking detailed timed information about behavior and exposure, are useful since they have a well-defined target population, often have good response rates, and are simpler and less expensive to conduct than prospective studies. This paper reviews retrospective TTP studies from a methodological viewpoint and shows how methodological problems can be avoided or minimized by appropriate study design, conduct, and analysis. Sensitivity analyses using data from four European retrospective TTP studies are presented to explore the issues. Although the identified biases tend to have small impacts, the effects are not systematic across studies, and sensitivity analyses are recommended routinely. Planning bias can be checked by comparing propensity to report contraceptive failures in different exposure groups. Medical intervention bias can be avoided by censoring and inclusion of unsuccessful pregnancy attempts. Truncation bias can be a serious problem if unrecognized, but it is avoidable with appropriate study design and/or analysis. Behavior change bias can be minimized by assessing the covariates at the beginning of unprotected intercourse. More complete inference is possible if the study design covers the whole population, not just those who achieve a pregnancy.
data collection; fertility; infertility; questionnaires; reproduction
Abbreviations: TTP, time to pregnancy
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