Figure 1.

From Bonde et al. (1998) Lancet: 430 couples with no previous reproductive experience, aged 20–35 years participated in a study on association between semen quality and the probability of conception in a single menstrual cycle. The couples discontinued use of contraception, and were followed up for six menstrual cycles or until a pregnancy was verified within this period. Each man was asked to provide a semen sample at enrolment. Women kept a daily record of vaginal bleeding and sexual activity. The association between semen quality and likelihood of pregnancy was assessed by logistic regression, adjusted for sexual activity and female factors associated with low fertility. There were 256 (59.5%) pregnancies among the 430 couples: 165 (65.0%) among those with a sperm concentration of 40 mill/mL or more, and 84 (51.2%) among those with lower sperm concentrations. The probability of conception increased with increasing sperm concentration up to 40 mill/mL, but any higher sperm density was not associated with additional likelihood of pregnancy.