Are volunteers delivering semen samples in fertility studies a biased population?

Background: It is not known to what extent the results of epidemiological studies on male fertility and semen quality based on voluntary participation in the general population are relevant. Methods: In a study on the reproductive health of male partners of pregnant women, information was obtained from a group of men agreeing to collect a semen sample and to complete a questionnaire (group A), a group only completing the questionnaire (group B) and from men refusing to participate altogether (group C). Results: The participation rate (group A) was 15.8% for 1409 men approached. Ages and socio-professional status were similar in the three groups. Time to pregnancy (TTP) was not significantly different in groups A and B, although there appeared to be an insignificantly higher proportion of couples taking longer than 12 months to conceive in group A than in group B. A history of urogenital disease appeared to be more frequent in groups A and B than in the general population. However, comparable semen characteristics were found for men with or without a history of urogenital disease. Pregnancy outcomes were similar in the three groups. Conclusions: The present study does not suggest major selection bias, although the social and reproductive histories of these men may prompt them to participate. Such factors need to be accounted for in similar studies.