title: Multilevel model to assess sources of variation in follicular growth close to the time of ovulation in women with normal fertility: a multicenter observational study.
abstract: BACKGROUND
  To assess the amount of variability in ovarian follicular growth rate and maximum follicular diameter related to different centers, women and cycles of the same women in a multicenter observational study of follicular growth.
  
  
  METHODS
  Secondary analysis of a prospective cohort study from eight centers in Europe. There were 533 ultrasound examinations in 282 cycles of 107 women with normal fertility. A random effects model with center, woman and cycle as hierarchical units of variation was used to analyze mean follicular diameter on days preceding ovulation.
  
  
  RESULTS
  Follicular growth did not differ by center. There was homogenous growth across women and cycles, and the maximum follicular diameter before ovulation varied substantially across cycles but not across women. Many (about 40%) women had small maximum follicular diameter on the day before ovulation (<19 mm). Pre-ovulatory cycle length was not related to maximum follicular diameter.
  
  
  CONCLUSIONS
  In normal fecundity, there is a substantial variation in maximum follicular diameter from cycle to cycle based on variation in the duration of follicular development, but the variation could not be explained by different characteristics of different women. Explanation of variation in follicular growth has to be found on the cycle level.
authors.affiliation: Department of Public Health Medicine, School of Public Health, University of Bielefeld, Bielefeld, Germany. rmikolajczyk@uni-bielefeld.de
authors.forename: Rafael T
authors.initials: RT
authors.lastname: Mikolajczyk
authors.forename: Joseph B
authors.initials: JB
authors.lastname: Stanford
authors.forename: René
authors.initials: R
authors.lastname: Ecochard
mesh_terms: D000293:Adolescent; D000328:Adult; D000704:Analysis of Variance; D015331:Cohort Studies; D005260:Female; D005498:Follicular Phase / Q000502:physiology; D006801:Humans; D008875:Middle Aged; D006080:Ovarian Follicle / Q000000981:diagnostic imaging / Q000254:growth & development* / Q000502:physiology; D010060:Ovulation; D011446:Prospective Studies; D013997:Time Factors; D014463:Ultrasonography