Ratio of boys to girls at birth

doi:10.1136/bmj.39141.622917.80

BMJ 2007;334:486-487 (10 March), doi:10.1136/bmj.39141.622917.80

Editorials

Ratio of boys to girls at birth

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

The first 150 words of the full text of this article appear below.

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

In this week's BMJ, Joffe and colleagues⁴ pool data from severallarge fecundity studies, further confirming the lack of associationbetween secondary sex ratio (boys to girls at birth) and timeto pregnancy. On this point, we are confident the authors arecorrect. However, their hypothesis that the sex ratio couldbe a marker of adverse effects on the male reproductive systemis less certain. They suggest that a slight decline over timein the secondary sex ratio . . . [Full text of this article]

Jens Peter Ellekilde Bonde, professor of occupational medicine¹, Allen Wilcox, senior investigator²

¹ Department of Occupational Medicine, Aarhus University Hospital, DK 8000 Aarhus, Denmark, ² Epidemiology Branch, National Institute of Environmental Health Sciences, Durham, North Carolina, USA

jpbon@as.aaa.dk

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