TIMING AND PATTERNS OF EXPOSURES DURING PREGNANCY AND THEIR IMPLICATIONS FOR STUDY METHODS

Patterns of exposure variability across pregnancy were examined for me dical, lifestyle, residential, and occupational exposures in a populat ion-based sample of 357 livebirths from 10 rural California counties. A new measure of variability, the ratio of overall prevalence to time- window-specific prevalence, is introduced. The higher the overall:time window (OTW) ratio, the greater the potential for misclassification w hen using anytime-during-pregnancy prevalence for an agent that exerts its effect in a smaller time window, Exposures to cigarettes, marijua na, saunas/hot tubs, factors related to location of residence, and som e workplace substances tended to be of longer duration, Intertrimester concordance was high (kappa's > 0.8) for smoking, residential proximi ty to crops, and use of video display terminals; moderately high (kapp a's between 0.4 and 0.8) for many occupational exposures; and low (kap pa's < 0.4) for illnesses, which tended to be of short duration, The l owest OTW ratios were for smoking and some residential exposures (1.1- 1.3), while OTW ratios were much higher for paint applications, influe nza, vaginal infections, and ultrasound (reaching, e.g., 4-6), Use of anytime-during-pregnancy exposure indices can bias measures of associa tion between risk factors and adverse pregnancy outcomes, particularly if the OTW ratio is high, Misclassification bias occurs if there is a vulnerable time window during which the exposure exerts its effect. T he misclassification can be differential when the average length of ge station of cases is shorter than that of controls. For exposures that vary, investigations of pregnancy outcome should collect as much detai l as feasible regarding timing.