title: [Why do some women only give birth to boys or to girls?].
abstract: BACKGROUND
  There are more families with many children of only one sex than can be explained by unconditional probability.
  
  
  METHODS
  Data from the Medical Birth Registry of Norway, covering more than 540,000 women with two, three and four births for the period 1967-2003, were used to study how the sex distributions of children already born affected the probability of a new birth and the sex of the next sibling.
  
  
  RESULTS
  Women with two children of the same sex had a higher probability of having more children compared to women with two children of both sexes (RR=1.14, (1.14-1.15)). This also applied to mothers with three children (RR=1.15 (1.13-1.17)). The probability was highest for mothers with boys only. Multiple births and parity affected the probability of giving birth to a boy versus a girl, but the sex composition of already born siblings had no influence.
  
  
  CONCLUSIONS
  We have no evidence that some parents may have a probability of having a boy versus a girl that differs from the rest of the population. The fact that there are more siblings of only one sex is mainly behavioural; some mothers with only girls or only boys keep having more children in an attempt to receive a child of the opposite sex.
authors.affiliation: Det medisinske fakultet, Norges teknisk-naturvitenskapelige universitet, 7489 Trondheim. lippert@stud.ntnu.no
authors.forename: Tonje
authors.initials: T
authors.lastname: Lippert
authors.forename: Rolv
authors.initials: R
authors.lastname: Skjaerven
authors.forename: Kjell A
authors.initials: KA
authors.lastname: Salvesen
mesh_terms: D001723:Birth Rate; D005191:Family Characteristics; D005260:Female; D006801:Humans; D008297:Male; D009664:Norway; D010298:Parity; D011247:Pregnancy; D011336:Probability; D012042:Registries; D017678:Sex Distribution; D012737:Sex Factors; D012744:Sex Ratio; D012801:Sibling Relations