title: Sex ratio and time to pregnancy: analysis of four large European population surveys.
abstract: OBJECTIVE
  To test whether the secondary sex ratio (proportion of male births) is associated with time to pregnancy, a marker of fertility. Design Analysis of four large population surveys. Setting Denmark and the United Kingdom. Participants 49 506 pregnancies.
  
  
  METHODS
  Secondary sex ratio.
  
  
  RESULTS
  No association was found between the sex ratio and time to pregnancy and no discernible trend was found for sex ratio with time to pregnancy, either within individual datasets or in the pooled analysis. The odds ratios were 0.97 (95% confidence interval 0.90 to 1.04) for contraceptive failures, 1.01 (0.96 to 1.05) for time to pregnancy of 2-4 months, 1.02 (0.97 to 1.08) for 5-10 months, 0.98 (0.93 to 1.03) for 11 months or more, and 0.88 (0.74 to 1.06) for fertility treatment, with 0-1 months as the reference category.
  
  
  CONCLUSIONS
  No association was found between the secondary sex ratio and time to pregnancy.
authors.affiliation: Department of Epidemiology and Public Health, Imperial College, London W2 1PG. m.joffe@imperial.ac.uk
authors.forename: Mike
authors.initials: M
authors.lastname: Joffe
authors.forename: James
authors.initials: J
authors.lastname: Bennett
authors.forename: Nicky
authors.initials: N
authors.lastname: Best
authors.forename: Tina Kold
authors.initials: TK
authors.lastname: Jensen
mesh_terms: D005060:Europe; D005260:Female; D005298:Fertility / Q000502:physiology*; D006801:Humans; D011159:Population Surveillance; D011247:Pregnancy / Q000706:statistics & numerical data*; D012744:Sex Ratio; D013997:Time Factors