Human Reproduction, Vol. 14, No. 12, 3120-3125,
December 1999
© 1999 European Society of Human Reproduction and Embryology
Natural variation in the human sex ratio
Centre for Research in Health & Social Statistics, The Danish National Research Foundation, Sejrøgade 11, DK-2100 Copenhagen Ø, Denmark
Analysis of the effect of multiple birth, birth order, age of parents and the sexes of preceding siblings on the secondary sex ratio was performed for 815 891 children, born in Denmark, 1980–1993. The proportion of males was analysed as a function of multiple birth, birth order, age of parents and the sexes of preceding siblings, using contingency tables, 
2 tests and logistic regression analysis. The secondary sex ratio decreased with increased number of children per plural birth and with paternal age, whereas no independent effect was observed for maternal age, birth order, the sex of the preceding child, or the combination of sexes of previously born children in the family.
Key words: birth order/parental age/sex composition/sex ratio/twins
1 To whom correspondence should be addressed
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