Time to pregnancy and sex of offspring: cohort study

doi:10.1136/bmj.331.7530.1437

BMJ 2005;331:1437-1438 (17 December), doi:10.1136/bmj.331.7530.1437

Paper

Time to pregnancy and sex of offspring: cohort study

Luc J M Smits, lecturer¹, Rob A de Bie, professor¹, Gerard G Essed, professor², Piet A van den Brandt, professor¹

¹ Maastricht University, Department of Epidemiology, PO Box 616, 6200 MD Maastricht, Netherlands, ² University Hospital Maastricht, Department of Obstetrics and Gynaecology, Maastricht

Correspondence to: L J M Smits luc.smits@epid.unimaas.nl

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

Introduction

The proportions of X and Y chromosome bearing sperms in humansemen are equal, but more boys than girls are born. Male embryosand fetuses have a greater risk of attrition in utero than theirfemale counterparts, and therefore male excess is likely tobe still larger at the time of conception. It remains unexplained,however, what is responsible, presumably at some point betweeninsemination and conception, for the greater probability ofY bearing sperms fusing with the ovum. One hypothesis relatesto experiments showing that Y bearing sperms swim faster thanX bearing sperms in viscous fluids.¹ For natural conception,human sperms have to penetrate cervical mucus, the viscosityof which varies among and within women.² Since mucal viscosityalso influences the probability of conception,² we expectedthat natural conceptions that take longer to achieve are morelikely to be male than quick conceptions. We tested our predictionby . . . [Full text of this article]

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Time to pregnancy and sex of offspring: J�rn Olsen, et al.
bmj.com, 21 Dec 2005 [Full text]
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