title: Prognostic factors of first birth interval using the parametric survival models.
abstract: BACKGROUND
  In recent years, the study of birth intervals has been a main determinant of the levels of fertility in the populations, as it is associated with rates of fertility and population growth.
  
  
  OBJECTIVE
  The purpose of this study was to firstly apply survival analysis for modeling of first birth interval and secondly to explore its determinants.
  
  
  METHODS
  In a cross sectional study, the fertility history of 858 women was collected in rural areas of Shiraz (southern Iran) in 2008. We used the survival analysis such as cox regression and alternative parametric models to evaluate the prognostic factors of first birth interval.
  
  
  RESULTS
  Among the explanatory variables of interest, age at marriage, level of women's education, and menstrual status had highly significant effects on the duration of birth interval after marriage (p<0.01).
  
  
  CONCLUSIONS
  It is concluded that the suitable parametric models would be a useful tool for fitting to first birth interval, the fact that has been less paid attention to in researches.
authors.affiliation: Department of Biostatistics, Medical School, Shiraz University of Medical Sciences, Shiraz, Iran.
authors.forename: Zahra
authors.initials: Z
authors.lastname: Shayan
authors.affiliation: Department of Biostatistics, Medical School, Shiraz University of Medical Sciences, Shiraz, Iran.
authors.forename: Seyyed Mohammad Taghi
authors.initials: SM
authors.lastname: Ayatollahi
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: Department of Family Health and Population, Shiraz University of Medical Sciences, Shiraz, Iran.
authors.forename: Fariba
authors.initials: F
authors.lastname: Moradi
keywords: First birth interval; Parametric models; Survival analysis