title: The male disadvantage and the seasonal rhythm of sex ratio at the time of conception.
abstract: BACKGROUND
  In accordance with a presumed greater fragility of male versus female pregnancies, we tested whether sex ratio (male/female ratio) of vital pregnancies is higher in seasons more favourable for reproduction.
  
  
  METHODS
  A retrospective study was performed on 14,310 births which had occurred in our institute between 1995-2001. For each single pregnancy the time of conception was calculated by the last menstrual period recall and confirmed or redefined by ultrasound in 95.8% of cases. The sex ratio of 199,454 pregnancies which had occurred in the Modena County between 1936-1998 was also stratified according to the month of birth.
  
  
  RESULTS
  Sex ratio of institutional deliveries was 0.511 and was identical to that obtained from the County registry. Sex ratio at birth did not show a significant seasonal variation. By contrast, sex ratio calculated at time of conception showed a seasonal rhythm, with amplitude of 2.4% and peak values in October (confidence interval: +/-43 days). The rhythm was in phase with the rhythm of conception that showed peak values in September (confidence interval: +/-37 days) and an amplitude of 7%.
  
  
  CONCLUSIONS
  The superimposition of the phase of sex ratio and conception rhythms supports the contention that more males than females are conceived in seasons with more favourable reproductive conditions.
authors.affiliation: Department of Obstetrics Gynecology and Pediatric Sciences, Gynecology Unit, Policlinico of Modena, Via del Pozzo 71, 41100, Modena, Italy. cagnacci@unimore.it
authors.forename: A
authors.initials: A
authors.lastname: Cagnacci
authors.forename: A
authors.initials: A
authors.lastname: Renzi
authors.forename: S
authors.initials: S
authors.lastname: Arangino
authors.forename: C
authors.initials: C
authors.lastname: Alessandrini
authors.forename: A
authors.initials: A
authors.lastname: Volpe
mesh_terms: D005260:Female; D005306:Fertilization; D006801:Humans; D008297:Male; D008499:Medical Records; D010507:Periodicity; D011247:Pregnancy; D012042:Registries; D012189:Retrospective Studies; D012621:Seasons; D012744:Sex Ratio