title: [Expectancy of vacation as a determining factor of the annual rhythm of menarche].
abstract: BACKGROUND
  Our previous studies have shown that the monthly rhythm of menarche is associated to the expectancy of holidays or vacations. If this hypothesis is true, menarche episodes in Chile should cluster in December, the month prior to summer vacations.
  
  
  OBJECTIVE
  To study whether the expectancy of holidays or vacations is the main determinant of the annual rhythm of menarche.
  
  
  METHODS
  During 1990 and 1991, a questionnaire about the date of birth and date of menarche, was applied to girls living in Santiago (Chile) and Medellin (Colombia), aged 8 to 18.5 years. They were requested to register the year, month or day of menarche, only if they recalled it precisely.
  
  
  RESULTS
  Discarding clerical errors, data was obtained from 3,225 Chilean and 3,435 Colombian girls. Of these 73 and 63% of Chilean and Colombian girls recalled the day of menarche. Among Chilean girls an excess of menarche episodes was observed in December, January and February. The mean age per month of menarche reached its highest value of 155.6+/-15 months in December and fell to 148+/-17 months in January (p <0.001). Among Colombian girls, mean age per month of menarche was also higher in December and lower in January (152+/-15 and 147+/-14 months respectively, p <0.001). However a peak of 152+/-15 months was also observed in June.
  
  
  CONCLUSIONS
  A similar distribution of menarche episodes was observed among Chilean and Colombian girls, with a peak in the months preceding holidays. The influence of the photoperiod is unlikely since Medellin is located in parallel 6 and there are little variations in daylight exposure in the different seasons. Therefore, our data supports the hypothesis that the prospect of holidays is a determinant of menarche.
authors.affiliation: Programa de Genética Humana, ICBM, Facultad de Medicina, Universidad de Chile, Santiago, Chile. cvalenzu@med.uchile.cl
authors.forename: Carlos Y
authors.initials: CY
authors.lastname: Valenzuela
mesh_terms: D000293:Adolescent; D000704:Analysis of Variance; D002648:Child; D002677:Chile; D003105:Colombia; D016001:Confidence Intervals; D004644:Emotions / Q000502:physiology*; D005260:Female; D006801:Humans; D007899:Leisure Activities / Q000523:psychology*; D008572:Menarche / Q000502:physiology* / Q000523:psychology; D012621:Seasons; D011795:Surveys and Questionnaires; D013997:Time Factors