title: The study of waiting time to first pregnancy in the south of Iran: A parametric frailty model approach.
abstract: BACKGROUND
  Time to first pregnancy (TTFP) has never been studied in an Iranian setting. Lifestyle, occupational and environmental factors have been suggested to affect the female reproduction.
  
  
  OBJECTIVE
  This study was conducted to measure TTFP in the south of Iran and survey the effects of several similar factors on TTFP by frailty models.
  
  
  METHODS
  The data on TTFP were available for 882 women who were randomly selected from the rural population (the south of Iran). Only the first and the planned pregnancies of every woman were included. The data were collected retrospectively by using self-administered questionnaires. Frailty and shared frailty models were used to determine which factors had the highest impact on TTFP.
  
  
  RESULTS
  The median TTFP was 6.4 months and several factors were surveyed. However, only the age of marriage, height, maternal education and regularity of menstruation prior to conception were selected in the multivariable models.
  
  
  CONCLUSIONS
  Among the several factors which were included in the study, the result of frailty model showed that the height, age of marriage and regular menstruation seemed more notable predictors of TTFP.
authors.affiliation: Department of Biostatistics, Infertility Research Center , Shiraz University of Medical Sciences, Shiraz, Iran.
authors.forename: Najaf
authors.initials: N
authors.lastname: Zare
authors.affiliation: Social Determinants of Health Research Center, Kurdistan University of Medical Sciences, Sanandaj, Iran.
authors.forename: Bijan
authors.initials: B
authors.lastname: Nouri
authors.affiliation: Family Health Unit , The Office of Vice Chancellor for Health Affairs , Shiraz University of Medical Sciences , Shiraz, Iran.
authors.forename: Fariba
authors.initials: F
authors.lastname: Moradi
authors.affiliation: Social Determinants of Health Research Center, Kurdistan University of Medical Sciences, Sanandaj, Iran.
authors.forename: Maryam
authors.initials: M
authors.lastname: Parvareh
keywords: Fertility; Frailty models; Survival models; Time to pregnancy