title: Clustering of retrospectively reported and prospectively observed time-to-pregnancy.
abstract: OBJECTIVE
  Given reportedly high clustering but limited validity of retrospectively reported time-to-pregnancy (TTP), we assessed within-woman clustering for retrospectively reported TTPs alone and including gold-standard prospectively observed TTPs among women with 2 or more retrospectively reported and 1 or more prospectively observed TTPs. We further investigated whether past trying times inform future trying time among women with 1 or more retrospectively reported and 1 or more prospectively observed TTPs.
  
  
  METHODS
  Five hundred one couples attempting pregnancy were prospectively observed until human chorionic gonadotropin pregnancy or 12 months of trying. Women reported TTP for past planned pregnancies. Clustering as measured by the frailty variance was estimated using discrete Cox frailty models, adjusted for age, body mass index, smoking at each attempt. Utility of past attempts to inform future attempts was assessed with discrete Cox models and relative risk regression, adjusted for enrollment age, body mass index, smoking.
  
  
  RESULTS
  Seventy-five women with 2 or more prior pregnancies contributed 180 retrospective and 91 prospective TTPs for frailty modeling. Retrospectively reported TTP clustering was high (frailty variance = 0.89) but substantially lower when including prospectively observed TTPs (frailty variance = 0.42). Among 202 women with 1 or more prior pregnancies, past trying times did not inform future trying time.
  
  
  CONCLUSIONS
  TTP recall rather than TTP may account for clustering. Past trying times may not inform future trying times.
authors.affiliation: Division of Intramural Population Health Research, Eunice Kennedy Shriver National Institute of Child Health and Human Development, Rockville, MD. Electronic address: katherine.sapra@nih.gov.
authors.forename: Katherine J
authors.initials: KJ
authors.lastname: Sapra
authors.affiliation: Department of Epidemiology and Biostatistics, Arnold School of Public Health, University of South Carolina, Columbia, SC.
authors.forename: Alexander C
authors.initials: AC
authors.lastname: McLain
authors.affiliation: Glotech, Inc., Rockville, MD.
authors.forename: José M
authors.initials: JM
authors.lastname: Maisog
authors.affiliation: Division of Intramural Population Health Research, Eunice Kennedy Shriver National Institute of Child Health and Human Development, Rockville, MD.
authors.forename: Rajeshwari
authors.initials: R
authors.lastname: Sundaram
authors.affiliation: Division of Intramural Population Health Research, Eunice Kennedy Shriver National Institute of Child Health and Human Development, Rockville, MD.
authors.forename: Germaine M
authors.initials: GM
authors.lastname: Buck Louis
mesh_terms: D000293:Adolescent; D000328:Adult; D016000:Cluster Analysis; D005260:Female; D006801:Humans; D011247:Pregnancy / Q000706:statistics & numerical data*; D011446:Prospective Studies; D012189:Retrospective Studies; D013997:Time Factors; D061685:Time-to-Pregnancy; D014481:United States; D055815:Young Adult
keywords: Conception; Conception delay; Fecundity; Time-to-pregnancy