Misclassification of gestational age in the study of spontaneous abortion.

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.