Ratio of boys to girls at birth

BMJ 2007; 334 doi: https://doi.org/10.1136/bmj.39141.622917.80 (Published 08 March 2007) Cite this as: BMJ 2007;334:486
  1. Jens Peter Ellekilde Bonde, professor of occupational medicine1,
  2. Allen Wilcox, senior investigator2
  1. 1Department of Occupational Medicine, Aarhus University Hospital, DK 8000 Aarhus, Denmark
  2. 2Epidemiology Branch, National Institute of Environmental Health Sciences, Durham, North Carolina, USA
  1. jpbon{at}as.aaa.dk

    Is not related to the time taken to conceive, or exposure to environmental agents

    In 2005, the BMJ published a paper suggesting that subfertile couples were more likely than fertile couples to have boys.1 This observation was offered as support of the hypothesis that sperm bearing Y chromosomes swim faster through viscous cervical mucous. In the flurry of letters that followed, other researchers reported that their data did not support the sex ratio finding.2 Furthermore, the sperm swimming hypothesis was exposed as a persistent myth.3

    In this week's BMJ, Joffe and colleagues4 pool data from several large fecundity studies, further confirming the lack of association between secondary sex ratio (boys to girls at birth) and time to pregnancy. On this point, we are confident the authors are correct. However, their hypothesis that the sex ratio could be a marker of adverse effects on the male reproductive system is less certain. They suggest that a …

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