BMJ  2007;334:524 (10 March), doi:10.1136/bmj.39097.508426.BE (published 2 February 2007)

Research

Sex ratio and time to pregnancy: analysis of four large European population surveys

Mike Joffe, emeritus reader1, James Bennett, research associate1, Nicky Best, professor of statistics and epidemiology1, Tina Kold Jensen, professor of environmental epidemiology2

1 Department of Epidemiology and Public Health, Imperial College, London W2 1PG , 2 University of Southern Denmark, Institute of Public Health, Denmark

Correspondence to: M Joffe m.joffe{at}imperial.ac.uk

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.

Main outcome measure 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.

Conclusion No association was found between the secondary sex ratio and time to pregnancy.


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