Relation between semen quality and fertility: a population-based study of 430 first-pregnancy planners

BACKGROUND Semen analysis is part of the routine assessment of infertile couples. WHO defines a sperm concentration above 20x10(6) per mL seminal fluid as normal. We studied the association between semen quality and the probability of conception in a single menstrual cycle in Danish couples with no previous reproductive experience. METHODS In 1992-94, we invited 52,255 trades-union members aged 20-35 years, who lived with a partner and had no children to take part in the study; 430 couples agreed. 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 (which was analysed without freezing). 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. RESULTS There were 256 (59.5%) pregnancies among the 430 couples: 165 (65.0%) among those with a sperm concentration of 40x10(6)/mL or more and 84 (51.2%) among those with lower sperm concentrations. The probability of conception increased with increasing sperm concentration up to 40x10(6)/mL, but any higher sperm density was not associated with additional likelihood of pregnancy. The proportion of sperm with normal morphology was strongly related to likelihood of pregnancy independently of sperm concentration. Semen volume and motility were of limited value in pregnancy prediction. INTERPRETATION Our study suggests that the current WHO guidelines for normal semen quality should be used with caution. Some men with sperm counts above the lower limit of the normal range defined by WHO may in fact be subfertile.