The aim of this study was to evaluate the association between semen quality and the secondary sex ratio (SSR),defined as the ratio of male to female live births.Our study cohort comprised 227 male partners who were enrolled prior to conception in Michigan and Texas between 2005 and 2009,and prospectively followed through delivery of a singleton birth.The male partners provided a baseline and a follow-up semen sample a month apart.Semen analysis was conducted to assess 27 parameters including five general characteristics,six sperm head measures,14 morphology measures,and two sperm chromatin stability assay measures.Modified Poisson regression models with a robust error variance were used to estimate the relative risk (RR) and 95％ confidence interval (95％ Cl) of a male birth for each semen parameter,after adjusting for potential confounders.Of the 27 semen parameters,only the percentage of bicephalic sperm was significantly associated with the SSR (2nd Vs 1st quartile,RR,0.65,95％ Cl,0.45-0.95,P＝ 0.03;4th vs 1st quartile,RR,0.61,95％ Cl,0.38-1.00,P＜ 0.05 before rounding to two decimal places),suggestive of a higher percentage of bicephalic sperm being associated with an excess of female births.Given the exploratory design of the present study,this preconception cohort study suggests no clear signal that human semen quality is associated with offspring sex determination.