title: Reducing misclassification in assignment of timing of events during pregnancy.
abstract: BACKGROUND
  Perinatal epidemiology studies often collect only the calendar month in which an event occurs in early pregnancy because it is difficult for women to recall a specific day when queried later in pregnancy or postpartum. Lack of day information may result in incorrect assignment of completed gestational month because calendar months and pregnancy months are not aligned.
  
  
  METHODS
  To examine the direction and magnitude of misclassification, we compared 3 methods for assignment of completed gestational month: 1) calendar month difference, 2) conditional month difference, and 3) imputed month midpoint. We used data from the Pregnancy, Infection, and Nutrition Study for simulations.
  
  
  RESULTS
  Calendar month difference misclassified 54% of events as 1 month later in pregnancy compared with the actual completed month of gestation. Each of the other 2 methods misclassified approximately 12% of events to 1 month earlier and 12% to 1 month later.
  
  
  CONCLUSIONS
  Calendar month difference, a common method, has the greatest misclassification. Conditional month difference and imputed month midpoint, which require little effort to implement, are superior to calendar month difference for reducing misclassification.
authors.affiliation: Departments of Epidemiology, School of Public Health, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA. juanokyang@yahoo.com
authors.forename: Juan
authors.initials: J
authors.lastname: Yang
authors.forename: Katherine E
authors.initials: KE
authors.lastname: Hartmann
authors.forename: Amy H
authors.initials: AH
authors.lastname: Herring
authors.forename: David A
authors.initials: DA
authors.lastname: Savitz
mesh_terms: D015331:Cohort Studies; D005260:Female; D005865:Gestational Age; D006801:Humans; D009657:North Carolina; D049590:Postpartum Period; D011247:Pregnancy; D011446:Prospective Studies; D013533:Gestational Carriers; D011795:Surveys and Questionnaires; D013997:Time Factors