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American Journal of Epidemiology Advance Access published online on September 19, 2006
American Journal of Epidemiology, doi:10.1093/aje/kwj327
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American Journal of Epidemiology Copyright © 2006 by the Johns Hopkins Bloomberg School of Public Health All rights reserved; printed in U.S.A.
Received October 31, 2005
Accepted April 28, 2006

ORIGINAL CONTRIBUTIONS

Misclassification of Gestational Age in the Study of Spontaneous Abortion

Penelope P. Howards 1 *, Irva Hertz-Picciotto 2, Clarice R. Weinberg 3, and Charles Poole 4

1 Division of Epidemiology, Statistics, and Prevention Research, National Institute of Child Health and Human Development, Bethesda, MD
2 Department of Public Health Sciences, Divisions of Environmental and Occupational Health and Epidemiology, University of California, Davis, CA
3 Division of Intramural Research, National Institute of Environmental Health Sciences, Research Triangle Park, NC
4 Department of Epidemiology, University of North Carolina, Chapel Hill, NC

* To whom correspondence should be addressed.
Penelope P. Howards, E-mail: howardsp{at}mail.nih.gov


  Abstract

Most studies of spontaneous abortion are subject to left truncation, because conception is not observed and thus pregnant women are enrolled postconception. Cox regression can account for left truncation but uses gestational age data, which may be inaccurate. Dating is affected by reporting errors and variability in the day of ovulation. These errors may be differential by outcome, because gestational ages are more likely to be clinically corrected in continuing pregnancies than in pregnancies ending in spontaneous abortion. Errors may be differential by exposure status as well, if exposures affect the time of ovulation. The authors designed a simulation to examine bias caused by errors in gestational age. Pregnancies were assigned true and alternative gestational ages using different assumptions about random reporting error and error due to variation in the time between the last menstrual period and ovulation. In separate scenarios, the errors were differential by outcome, differential by exposure, differential by both exposure and outcome, or nondifferential. Hazard ratios were compared using accurate versus erroneous gestational ages. For proportional hazards, bias was only introduced when the error in gestational age was differential by exposure status. Bias was greatest when the magnitude of error for pregnancies at higher risk was much larger than that for pregnancies at lower risk.

Keywords: abortion; spontaneous; bias (epidemiology); gestational age; pregnancy; proportional hazards models.
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